Add instructions for publishing

PUBLISHING.md

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+# Publishing to crates.io
+
+Publishing `compiler-builtins` to crates.io takes a few steps unfortunately.
+It's not great, but it works for now. PRs to improve this process would be
+greatly appreciated!
+
+1. Make sure you've got a clean working tree and it's updated with the latest
+   changes on `master`
+2. Edit `Cargo.toml` to bump the version number
+3. Commit this change
+4. Run `git tag` to create a tag for this version
+5. Delete the `libm/Cargo.toml` file
+6. Comment out the `[dev-dependencies]` section of `Cargo.toml`
+7. Run `cargo +nightly publish --allow-dirty`
+8. Push the tag
+9. Push the commit
+10. Undo changes to `Cargo.toml` and the `libm` submodule