Add documentation about contributing to Willow

Dockerfile

@@ -20,3 +20,9 @@ RUN pip install --upgrade pip \
 COPY ./wagtail /code/wagtail/
 RUN cd /code/wagtail/ \
     && pip install -e .[testing,docs]
+
+# Install Willow from the host. This folder will be overwritten by a volume mount during run time (so that code
+# changes show up immediately), but it also needs to be copied into the image now so that Willow can be pip install'd.
+COPY ./libs/willow /code/willow/
+RUN cd /code/willow/ \
+    && pip install -e .[testing]

README.md

@@ -133,6 +133,12 @@ git remote add upstream git@github.com:wagtail/wagtail.git
 git pull --all
 ```
 
+Contributing to Willow
+----------------------
+
+You can use the same setup to contribute to Willow.
+You simply do the same operations to fork the Willow project and point your local copy of Willow to your fork.
+
 
 TODO
 ----