CONTRIBUTING

rust-chg: add project skeleton

This directory will host the reimplementation of cHg in Rust. It will use
Tokio [1], and tokio-hglib [2] which I wrote for an oxidized CHg, no idea
if there's such carbon bonding in nature btw.

 [1]: https://tokio.rs/
 [2]: https://bitbucket.org/yuja/tokio-hglib/

The reasoning for depending on Tokio is that it will allow us to handle Unix
signals in a safer way. Well, I believed that until I found a weird function,
handlestopsignal(), in cHg codebase. It resends the same signal to the same
process by temporarily masking the handler, which can't be inherently async.
So the signal handlers will stay in C, which means there isn't actually much
reason to write async code right now, other than I've already done most of
the async stuff, and slightly easier pager handling.

The reasoning for the rewrite is that it will eventually be possible to port
server-side config validation back to the client side, which will reduce
the complexity of the current daemon management. It will also encourage us
to write frontend library (e.g. command line and config parsers) in Rust.

The license is GPL2+ because it's likely to include derived work from the
cHg of C. The rust/chg crate is excluded from the root workspace as it's
unclear how the whole rust packages should be laid out. That can be revisited
later.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).