Added index to FAQ

FAQ.md

@@ -3,6 +3,14 @@
 If you have a question, please check the frequently asked questions below first.
 At the bottom you can find how to ask other questions, request features and report bugs.
 
+## Index
+
+* [Authorizing accounts](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-authorizing-accounts)
+* [Known problems](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-known-problems)
+* [Planned features](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-planned-features)
+* [Frequently requested features](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-frequently-requested-features)
+* [Frequently Asked Questions](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-frequently-asked-questions)
+* [Support](https://github.com/M66B/FairEmail/blob/master/FAQ.md#user-content-support)
 
 ## Authorizing accounts