Explain how to enable new Weboob backends in Flatisfy in CONTRIBUTING.md

CONTRIBUTING.md

@@ -45,5 +45,14 @@ locale.
   WIP commits into a single one, to maintain the invariant of "one feature, one
   commit".
 
-
 Thanks!
+
+
+## Adding support for a new Weboob backend
+
+To enable a new Weboob `CapHousing` backend in Flatisfy, you should add it to
+the list of available backends in
+[flatisfy/fetch.py#L69-70](https://git.phyks.me/Phyks/flatisfy/blob/master/flatisfy/fetch.py#L69-70)
+and update the list of `BACKEND_PRECEDENCES` for deduplication in
+[flatisfy/filters/duplicates.py#L24-31](https://git.phyks.me/Phyks/flatisfy/blob/master/flatisfy/filters/duplicates.py#L24-31).
+Thats' all!

flatisfy/filters/duplicates.py

@@ -20,7 +20,9 @@ from flatisfy import tools
 LOGGER = logging.getLogger(__name__)
 
 # Some backends give more infos than others. Here is the precedence we want to
-# use.
+# use. First is most important one, last is the one that will always be
+# considered as less trustable if two backends have similar info about a
+# housing.
 BACKENDS_PRECEDENCE = [
     "seloger",
     "pap",