Add patching instructions

PATCHING.md

@@ -0,0 +1,31 @@
+# Patching the plugin
+
+## Git workflow
+
+To patch an upstream release create a corresponding `releases/vX.Y.Z` branch in the CityScience
+fork, apply any changes e.g. cherry-pick commits from earlier release branches and then tag this
+branch. For example:
+
+```bash
+git clone [email protected]:CityScience/cypress-plugin-snapshots.git
+cd cypress-plugin-snapshots
+git remote add upstream [email protected]:meinaart/cypress-plugin-snapshots.git
+git fetch --all --tags
+git checkout -b releases/v1.2.9 v1.2.9
+# Apply fixes/patches and commit to release branch, get it reviewed etc.
+git tag v1.2.9-csc
+git push origin --tags
+```
+
+The tag name should match the corresponding tag from upstream, with `-csc` appended to it.
+
+## Testing
+
+To test a modified release:
+
+```bash
+npm install
+npm install --prefix cypress
+npm run ci:test
+```
+