Improvements to contributing.md, other minor changes

.github/funding.yml

@@ -0,0 +1 @@
+custom: "https://instaloader.github.io/contributing.html#donations"

.github/pull_request_template.md

@@ -1,7 +1,8 @@
 
 
 <!--
-Please describe:
+Thanks for your willingness to contribute to Instaloader! Please briefly
+describe
 
 - A motivation for this change, e.g.
   - Fixes # .