Add readthedocs config file

We need this to maintain our docs build, which won't work with shallow
clones.

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

.readthedocs.yaml

@@ -0,0 +1,6 @@
+---
+version: 2
+build:
+  jobs:
+    post_checkout:
+      - git fetch --unshallow