New 'Contributing' section on site

[konsoletyper]

Please, take a look at the site. I just added a new section for contributors. I suppose, some of you wanted to contribute code to TeaVM, but did not know where to start. This section provides hints. You can find section here: https://teavm.org/docs/contributing/general.html. Also, I added 'roadmap' marker to github issues, so now roadmap is just a set of issues, filtered by this marker.