Explain the use of external dependencies.

contributing.txt

@@ -20,6 +20,18 @@ following:
   a big feature)
 
 
+External dependencies
+
+The goal of Meson is to be as easily usable as possible. The user
+experience thould be "get Python3, run", even on
+Windows. Unfortunately this means that we can't have dependencies on
+projects outside of Python's standard library. This applies only to
+core functionality, though. For additional helper programs etc the use
+of external dependencies may be ok. If you feel that you are dealing
+with this kind of case, please raise the issue on the mailing list
+first.
+
+
 What not to contribute?
 
 There are a few things that people seem to want to add to Meson but which