add README.md (for gitea / github and the like)

README.md

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+gr-fosphor - gnuradio block for RTSA-like spectrum visualization
+================================================================
+
+This repository contains a GNU Radio block for RTSA-like spectrum visualization using OpenCL and OpenGL
+acceleration.
+
+
+The block has been used successfully on Linux, OSX and Windows.
+
+Homepage + Documentation
+------------------------
+
+For installation and usage guidelines please read the documentation available
+at <https://osmocom.org/projects/sdr/wiki/Fosphor>
+
+Forum
+-----
+
+We welcome any fosphor related discussions in the
+[SDR](https://discourse.osmocom.org/c/sdr/)
+section of the osmocom discourse (web based Forum).
+
+Mailing List
+------------
+
+Discussions related to libosmocore are happening on the
+osmocom-sdr@lists.osmocom.org mailing list, please see
+<https://lists.osmocom.org/mailman/listinfo/osmocom-sdr> for subscription
+options and the list archive.
+
+Please observe the [Osmocom Mailing List
+Rules](https://osmocom.org/projects/cellular-infrastructure/wiki/Mailing_List_Rules)
+when posting.
+
+Issue tracker
+-------------
+
+We are using the Osmocom redmine at <https://osmocom.org/projects/sdr/issues>
+
+Contributing
+------------
+
+We maintain our source code in a self-hosted instance of gitea at
+<https://gitea.osmocom.org/sdr/gr-fosphor.git>. You can send pull requests there, or send
+patches the old-fashioned way (`git send-email`) to the above-mentioned mailing list.