Bug 42308: Create README for tor-browser.

We drop the README.txt that comes from Mozilla Firefox and add README.md
for tor-browser.

README.md

+The tor-browser repository is based on Mozilla Firefox.
+
+See the following wiki links for more information:
+
++ [Repository Overview](https://gitlab.torproject.org/tpo/applications/team/-/wikis/Development-Information/Tor-Browser/Tor-Browser-Repository-Overview)
++ [Contributing](https://gitlab.torproject.org/tpo/applications/team/-/wikis/Development-Information/Tor-Browser/Contributing-to-Tor-Browser)

README.txt

-An explanation of the Firefox Source Code Directory Structure and links to
-project pages with documentation can be found at:
-
-    https://firefox-source-docs.mozilla.org/contributing/directory_structure.html
-
-For information on how to build Firefox from the source code and create the patch see:
-
-    https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html
-
-If you have a question about developing Firefox, and can't find the solution
-on https://firefox-source-docs.mozilla.org/, you can try asking your question on Matrix at chat.mozilla.org in `Introduction` (https://chat.mozilla.org/#/room/#introduction:mozilla.org) channel.
-
-
-Nightly development builds can be downloaded from:
-
-    https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/
-            - or -
-    https://www.mozilla.org/firefox/channel/desktop/#nightly
-
-Keep in mind that nightly builds, which are used by Firefox developers for
-testing, may be buggy.