Add notes explaining how to make a release

RELEASE.md

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+# Making a new release of git2cpp
+
+This covers making a new github release in the `git2cpp` repository, and propagating that release through to Emscripten-forge so that the new WebAssembly package is available in the Jupyterlite terminal.
+
+## Github release
+
+1. Submit and merge a `git2cpp` PR bumping the version number in `version.hpp`.
+2. Fetch the latest changes to your local `git2cpp` repo.
+3. Tag the new release using something like `git tag -a 0.0.2 -m "Version 0.0.2"`. Ideally sign the release by adding the `-s` flag.
+4. Push the new tag to the github repo using `git push upstream --tags`.
+5. Create a github release for the tag. In https://github.com/QuantStack/git2cpp:
+
+    - Click on `Releases`.
+    - Click on `Draft a new release`.
+    - Select the new tag in the `Choose a tag` dropdown.
+    - Click on `Generate release notes`. This will fill out the release notes based on the PRs merged since the last release.
+    - Edit the release notes if desired.
+    - Click on `Publish release`.
+
+## Update Emscripten-forge recipe
+
+The Emscripten-forge recipe at https://github.com/emscripten-forge/recipes needs to be updated with the new version number and SHA checksum. An Emscripten-forge bot runs once a day and will identify the new github release and create a PR to update the recipe. Wait for this to happen, and if the tests pass and no further changes are required, the PR can be approved and merged.
+
+After the PR is merged to `main`, the recipe will be rebuilt and uploaded to https://prefix.dev/channels/emscripten-forge-dev/packages/git2cpp, which should only take a few minutes.
+
+Any subsequent `cockle` or JupyterLite `terminal` deployments that are rebuilt will download and use the latest `git2cpp` WebAssembly package.