add conda-forge guide

Author: TomNicholas

HOW_TO_RELEASE.md

@@ -1,3 +1,5 @@
+### PyPi
+
 1. Make all the code changes, including updating version numbers in `pyproject.toml` and `README.md`
 
 2. Issue the release on github, creating a new tag
@@ -11,3 +13,13 @@
 6. Upload to pypi with `twine upload dist/*`
 
 7. Check that the [pypi package page](https://pypi.org/project/cubed-xarray/) shows the expected version.
+
+### Conda-forge
+
+1. Fork the [cubed-xarray feedstock](https://github.com/conda-forge/cubed-xarray-feedstock), creating a branch for the new version
+
+2. The branch should change the `recipe/meta.yaml`, specifically changing the package version number, resetting the build number to 0, and updating the hash
+
+3. The new hash is found using e.g. `openssl sha256 dist/cubed-xarray-0.0.5.tar.gz`
+
+4. Open a PR on the conda-forge feedstock for the changes, and merge it once the bots approve.