README: added mention of Poetry and how to use it

readme.md

@@ -2,3 +2,14 @@
 
 This project contains Attract, a task management extension for the Pillar
 platform.
+
+## Development
+
+Dependencies are managed via [Poetry](https://poetry.eustace.io/).
+
+```
+git clone git@git.blender.org:pillar-python-sdk.git ../pillar-python-sdk
+git clone git@git.blender.org:pillar.git ../pillar
+pip install -U --user poetry
+poetry install
+```