README.md in ffi-gdal-extensions-0.1.1 vs README.md in ffi-gdal-extensions-0.1.2

@@ -35,9 +35,36 @@
 release a new version, update the version number in `version.rb`, and then run
 `bundle exec rake release`, which will create a git tag for the version, push
 git commits and tags, and push the `.gem` file to
 [rubygems.org](https://rubygems.org).
 
+### Development Using Docker
+
+The `docker-compose.yml` file provides two configurations for running tests
+against:
+
+- `Dockerfile.gdal3`
+  - GDAL 3.4.2
+  - `liblwgeom` 2.5.5
+  - Ruby 3.1.1
+- `Dockerfile.gdal2`
+  - GDAL 2.4.4
+  - `liblwgeom` 2.5.5
+  - Ruby 3.1.1
+
+Running tests looks something like:
+
+...for GDAL3:
+
+```
+docker-compose run gdal3 bundle exec rake spec
+```
+
+...for GDAL2:
+
+```
+docker-compose run gdal2 bundle exec rake spec
+```
 ## Contributing
 
 Bug reports and pull requests are welcome on BitBucket at
 https://bitbucket.org/agrian/ffi-gdal-extensions. This project is intended to be
 a safe, welcoming space for collaboration, and contributors are expected to