Document config options for language

I recently searched high and low for a documentation of all the config options and didn't find anything. I believe this is a general problem but in my case I was interested specifically in the options for Java as I was generating with the swagger-codegen-maven-plugin. It's README says:

configOptions - a map of language-specific parameters (see below)

Yet, nowhere "below" are the options documented.

I thought about opening a PR to add the Java documentation (as follows) to the README. Maybe some guidance from @wing328 would be needed.

• sortParamsByRequiredFlag Sort method arguments to place required parameters before optional parameters. (Default: true)

• ensureUniqueParams
  Whether to ensure parameter names are unique in an operation (rename parameters that are not). (Default: true)

• allowUnicodeIdentifiers
  boolean, toggles whether unicode identifiers are allowed in names or not, default is false (Default: false)

• modelPackage
  package for generated models

• apiPackage
  package for generated api classes

• invokerPackage
  root package for generated code

• groupId
  groupId in generated pom.xml

• artifactId
  artifactId in generated pom.xml

• artifactVersion
  artifact version in generated pom.xml

• artifactUrl
  artifact URL in generated pom.xml

• artifactDescription
  artifact description in generated pom.xml

• scmConnection
  SCM connection in generated pom.xml

• scmDeveloperConnection
  SCM developer connection in generated pom.xml

• scmUrl
  SCM URL in generated pom.xml

• developerName
  developer name in generated pom.xml

• developerEmail
  developer email in generated pom.xml

• developerOrganization
  developer organization in generated pom.xml

• developerOrganizationUrl
  developer organization URL in generated pom.xml

• licenseName
  The name of the license

• licenseUrl
  The URL of the license

• sourceFolder
  source folder for generated code

• localVariablePrefix
  prefix for generated code members and local variables

• serializableModel
  boolean - toggle "implements Serializable" for generated models (Default: false)

• bigDecimalAsString
  Treat BigDecimal values as Strings to avoid precision loss. (Default: false)

• fullJavaUtil
  whether to use fully qualified name for classes under java.util. This option only works for Java API client (Default: false)

• hideGenerationTimestamp
  hides the timestamp when files were generated

• withXml
  whether to include support for application/xml content type and include XML annotations in the model (works with libraries that provide support for JSON and XML) (Default: false)

• dateLibrary Option. Date library to use

  • joda - Joda (for legacy app only)
  • legacy - Legacy java.util.Date (if you really have a good reason not to use threetenbp
  • java8-localdatetime - Java 8 using LocalDateTime (for legacy app only)
  • java8 - Java 8 native JSR310 (preferred for jdk 1.8+) - note: this also sets "java8" to true
  • threetenbp - Backport of JSR310 (preferred for jdk < 1.8)

• java8 Option. Use Java8 classes instead of third party equivalents

  • true - Use Java 8 classes such as Base64
  • false - Various third party libraries as needed

• useRxJava
  Whether to use the RxJava adapter with the retrofit2 library. (Default: false)

• useRxJava2
  Whether to use the RxJava2 adapter with the retrofit2 library. (Default: false)

• parcelableModel
  Whether to generate models for Android that implement Parcelable with the okhttp-gson library. (Default: false)

• usePlayWS
  Use Play! Async HTTP client (Play WS API) (Default: false)

• playVersion
  Version of Play! Framework (possible values "play24", "play25")

• supportJava6
  Whether to support Java6 with the Jersey1 library. (Default: false)

• useBeanValidation
  Use BeanValidation API annotations (Default: false)

• performBeanValidation
  Perform BeanValidation (Default: false)

• useGzipFeature
  Send gzip-encoded requests (Default: false)

• useRuntimeException
  Use RuntimeException instead of Exception (Default: false)

• library library template (sub-template) to use (Default: okhttp-gson)

  • jersey1 - HTTP client: Jersey client 1.19.4. JSON processing: Jackson 2.8.9. Enable Java6 support using '-DsupportJava6=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.
  • feign - HTTP client: OpenFeign 9.4.0. JSON processing: Jackson 2.8.9
  • jersey2 - HTTP client: Jersey client 2.25.1. JSON processing: Jackson 2.8.9
  • okhttp-gson - HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.8.1. Enable Parcelable models on Android using '-DparcelableModel=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.
  • retrofit - HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.3.1 (Retrofit 1.9.0). IMPORTANT NOTE: retrofit1.x is no longer actively maintained so please upgrade to 'retrofit2' instead.
  • retrofit2 - HTTP client: OkHttp 3.8.0. JSON processing: Gson 2.6.1 (Retrofit 2.3.0). Enable the RxJava adapter using '-DuseRxJava[2]=true'. (RxJava 1.x or 2.x)
  • resttemplate - HTTP client: Spring RestTemplate 4.3.9-RELEASE. JSON processing: Jackson 2.8.9
  • resteasy - HTTP client: Resteasy client 3.1.3.Final. JSON processing: Jackson 2.8.9
  • vertx - HTTP client: VertX client 3.2.4. JSON processing: Jackson 2.8.9
  • google-api-client - HTTP client: Google API client 1.23.0. JSON processing: Jackson 2.8.9

Start with modules/swagger-codegen-maven-plugin/README.md and review the existing configOptions section alongside the Java options listed in the issue. Document the Java-specific options and their defaults or allowed values, replacing the unresolved “see below” reference. Done means a Maven plugin user can find and understand the Java configOptions in the README.