add release instructions

RELEASE.md

@@ -0,0 +1,41 @@
+# Making a release
+
+To make a new release of bempp, follow the following steps:
+
+0) If you are yet to make a release on your current computer, run `cargo login` and copy an API
+   key from https://crates.io/me
+
+1) Checkout the `main` branch and `git pull`, then checkout a new branch called `release-v[x].[y].[z]`
+   (where `[x]`, `[y]`, and `[z]` are defined in the next step):
+   ```bash
+   git checkout main
+   git pull
+   git checkout -b release-v[x].[y].[z]
+   ```
+
+2) Update the version numbers in `Cargo.toml` and `c-api-tools-macros/Cargo.toml`.
+   The version numbers have the format `[x].[y].[z]`. If you are releasing a major
+   version, you should increment `[x]` and set `[y]` and `[z]` to 0.
+   If you are releasing a minor version, you should increment `[y]` and set `[z]`
+   to zero. If you are releasing a bugfix, you should increment `[z]`.
+
+3) Commit your changes and push to GitHub.
+
+4) Open a pull request and merge your changes in to `main`.
+
+5) [Create a release on GitHub](https://github.com/bempp/c-api-tools/releases/new) from the `main` branch.
+   The release tag and title should be `v[x].[y].[z]` (where `[x]`, `[y]` and `[z]` are as in step 2).
+   In the "Describe this release" box, you should bullet point the main changes since the last
+   release.
+
+6) Run `cargo publish --dry-run`, then run `cargo package --list` and
+   check that no unwanted extras have been included in the release.
+
+7) If everything is working as expected, run `cargo publish`. This will push the new version to
+   crates.io. Note: this cannot be undone, but you can use `cargo yank` to mark a version as
+   unsuitable for use.
+
+8) Open a pull request to `main` to update the version numbers in `Cargo.toml` and `pyproject.toml`
+   to `[x].[y].[z]-dev`
+
+9) Add the release to the next issue of [Scientific Computing in Rust Monthly](https://github.com/rust-scicomp/scientific-computing-in-rust-monthly)