repo: introduce AGENTS.md file

Resolves: https://redhat.atlassian.net/browse/NMT-2374

AGENTS.md

@@ -0,0 +1,34 @@
+- Study `CONTRIBUTING.md` to get all the key information on how to build
+  NetworkManager and write patches.
+- If the `origin` remote is pointed to GitHub, stop immediately and
+  remind the user that NetworkManager upstream development happens on
+  https://gitlab.freedesktop.org.
+- Ensure you have a `build/` meson directory that successfully builds
+  before commencing any work.
+- Before committing anything, make sure your code builds successfully,
+  passes all tests and is properly formatted.
+- Unless your work depends on previous unmerged work, make sure you are
+  on a new clean branch that is based on `main`.
+- If possible, update stale comments that become out of date due to your
+  changes, instead of outright removing them. This obviously does not
+  apply to TODO or FIXMEs that have been resolved and can be removed.
+- If the system has a installation of `podman`, you can use
+  `./contrib/scripts/nm-code-format-container.sh` to get a consistent
+  formatting of your changes. Otherwise, use `nm-code-format.sh` as
+  described in `CONTRIBUTING.md`.
+- Commit titles are written in the style of
+  `<area>: <short description>`. They should fit under 72 characters.
+  They should be concise and descriptive.
+- Please study existing commit messages to get an idea of how to write a
+  good commit message. If you edit a particular file, study its git
+  history to see how it was previously commented.
+- After making a commit, use `checkpatch.pl` as described by the
+  contributing document to ensure your patch is valid.
+- Unit tests live in this repository. Integration tests are in a
+  separate repository called `NetworkManager-ci`. You may have a clone
+  of it in ../NetworkManager-ci. Before making any changes to that repo,
+  please first study its contents and contributing guidelines.
+  When making a significant change, it might be good to add integration
+  tests for it. Whether to make unit tests or integration tests should
+  be decided on a case-by-case basis, depending on if the
+  surrounding/relevant code is already unit tested or not.