Need to document the basic requirements to support a language

#### What problem would the feature you're requesting solve? Please describe.
This issue is more about suggestions in documentation when releasing the BYO language feature.
To support a language in HttpWorker:
1. The language requires first-party or third-party Http libraries for parsing HttpWorkerRequests and generating HttpWorkerResponses.
2. If the custom language can be compiled into executable binary, make sure the program can be compiled into the destination platform (e.g. Windows x86).
3. If the custom language is an interpreted language, make sure the interpreter of the language supports the destination platform (e.g. Windows x86).
4. The platform in customer's devbox may be different to the platform supported on our cloud, please document our supported platforms.

| | Windows | Linux |
|----|---------------------------------------|-------------------------------------|
| V2 | Windows X86/X64 | Linux X64 (Debian Stretch) |
| V3 | Windows X86/X64 | Linux X64 (Debian Buster) |

No file or test is named. Start by locating the existing BYO language and HttpWorker documentation, then check how supported platforms are currently described. Done means documenting HTTP library requirements, compiled versus interpreted language constraints, and the Windows/Linux platform table from the issue.

Written by the indexing model from the issue text.