chore(docs): notes on unstable rustc in HACKING.md

Summary: Some notes to help guide potential contributors or lurkers
who are interested. Also has some notes on my use of tagref to keep
track of this stuff.

The goal is to not sound too passive, but also not overbearing, either.

GitHub Issue: facebook#265

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Change-Id: Itryyrvmlvxmssvxmwkxlrpyulvlwmlyq

HACKING.md

@@ -115,3 +115,37 @@ have written. Some rules:
 - Lists should use square brackets, e.g. ``Available targets: [`aa`, `bb`]``.
 - Error messages should start with an upper case letter. Error messages should
   not end with a period.
+
+### Unstable `rustc` features
+
+<!-- [tag:unstable-rustc] -->
+
+There is some effort to move away from using unstable Rust features in the
+codebase; the primary motivator for this is to ease consumption by third
+parties, who will often not allow unstable `rustc` toolchains in their projects,
+see https://github.com/facebook/buck2/issues/265 for some discussion and
+examples.
+
+There are other concerns as well, such as compiler stability, but the primary
+motivator is to meet users where they are at.
+
+In general, unstable feature usage is currently allowed, and there is currently
+NO commitment to keep the codebase working with the stable `rustc` release. If
+such a thing happens, it will be decided and &mdash; most likely, very loudly
+&mdash; communicated and enforced.
+
+If you want to build buck2 with a stable Rust compiler, please help out and
+submit patches. If you are _not_ concerned with this, then carry on as usual,
+though it would be nice to avoid too many gratuitous unstable features if you
+can.
+
+**NOTE**: If you want to look around the codebase for notes on unstable feature
+usage, and to keep track of progress, please keep track of issue #265 above; you
+can also use the **[tagref]** tool to search for references to this task in the
+codebase with the following command and learn more or help:
+
+[tagref]: https://github.com/stepchowfun/tagref
+
+```bash
+tagref list-refs | grep ref:unstable-rustc
+```