Provide documenation on contributing.

I've just raised a pull request with an enhancement to json_serialization.

Getting a development environment setup with rather difficult and a document that provides guidance on how to get started would be rather helpful.

As part of the PR i've pushed a number of alternate pubspec.yaml files.
`pubspec-dev.yaml'.

These version included dependency overrides to allow the three core projects to find each other on the local file system.

This got me around most of the problems but I was still unable to get my application to run with json_serialization on the local path.

The specific problem is a resolving dependancies.
build_config seems to be the driver of the problem...
```
The current Dart SDK version is 2.5.0.

Because build_runner >=1.7.2 requires SDK version >=2.6.0-dev.8.0 <3.0.0 and no versions of build_runner match >1.7.1 <1.7.2, build_runner >1.7.1 is forbidden.

(1) So, because build_runner 1.7.1 depends on build_config >=0.4.1 <0.4.2, build_runner >=1.7.1 requires build_config >=0.4.1 <0.4.2.
Because no versions of build_config match >0.4.1+1 <0.4.2 and build_config >=0.3.1+4 <=0.4.1 depends on json_annotation >=1.0.0 <3.0.0, build_config >=0.3.1+4 <0.4.1+1 or >0.4.1+1 <0.4.2 requires json_annotation >=1.0.0 <3.0.0.

And because build_config 0.4.1+1 depends on json_annotation >=1.0.0 <4.0.0, build_config >=0.3.1+4 <0.4.2 requires json_annotation >=1.0.0 <4.0.0.

And because build_runner >=1.7.1 requires build_config >=0.4.1 <0.4.2 (1), build_runner >=1.7.1 requires json_annotation from hosted.

So, because squarephone depends on both json_annotation from path and build_runner ^1.7.1, version solving failed.
pub get failed (1)
exit code 1
```
I've tried a number of variations using the classic 'any' version to get a resolution but nothing works.

Any assistance in getting a dev environment working would be greatly appreciated.