add developer notes

[GitMensch]

I suggest to move what we have in README.md (which is shown to a user of the extension) to a HACKING.md, possibly under "src" and heavily extend this, at least including a note what files are for what.

In general it would be nice to have multi-line comments in the .ts files, I suggest a short header providing the overview "this file contains code for xyz, is triggered by xyz2 and xyz3" and at least include a short ts-doc (I guess something like javadoc exists ...) for each function and global variables.

Yes - the code is not that big currently, but for someone to start it still takes longer than necessary. "Believing in the community" for PR is a reasonable and good thing, to make that happen means to either provide good documentation or extended mentoring (and silly questions on the tracker).