add some info wrt f-droid handling

docs/f-droid.md

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+# F-Droid - Overview
+
+- https://gitlab.com/fdroid/fdroiddata/blob/master/metadata/com.b44t.messenger.txt
+  contains the descripion and all meta data shown for Delta Chat on F-Droid
+
+- when a new tag in the form `v1.2.3` is added to the deltachat-android repo,
+  F-Droid will pick up this version and distribute it - 
+  this may take some days or even weeks.
+  
+  before creating a new version, please have a look at docs/release-checklist.md
+
+
+# Use F-Droid-tools locally
+
+$ git clone https://gitlab.com/fdroid/fdroiddata
+$ git clone https://gitlab.com/fdroid/fdroidserver
+$ cd fdroiddata
+$ ../fdroidserver/fdroid checkupdates -v com.b44t.messenger
+
+(via https://f-droid.org/docs/Installing_the_Server_and_Repo_Tools/ 
+and https://f-droid.org/docs/Building_Applications/ -
+might require `pip install pyasn1 pyasn1_modules pyaml`)
+
+
+# Changing the Description on F-Droid
+
+- the description can be changed via a PR to the file above
+
+- make sure there is a "newline" at the end of the description
+  (see https://gitlab.com/fdroid/fdroiddata/merge_requests/3580)
+
+- reformat the metadata using
+  $ ../fdroidserver/fdroid rewritemeta com.b44t.messenger  # called from fdroiddata dir
+ 

docs/playstore.md

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+TODO: add information about whats important for Delta wrt Google Play here.
+also a Playstore-Release-Checklist is required in `release-checklist.md`