Add README.md file

Change-Id: I65ebd735d3c99c47b8f1a88c5a7eb272635d52c3
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A README.md
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+= Osmocom DIAMETER -> GSUP translator =
+
+This project implements a proxy translating Diameter into GSUP, hence allowing
+Diameter clients connecting to GSUP server. This allows, for instance, using
+OsmoHLR as an HSS used by 4G/5G nodes.
+
+== Building ==
+
+Install erlang and rebar3 packages (not "rebar", that's version 2! You may need
+to compile it from source in some distros).
+
+$ rebar3 compile
+$ rebar3 escriptize
+
+== Testing ==
+
+Unit tests can be run this way:
+$ rebar3 eunit
+
+== Running ==
+
+Once osmo_dia2gsup is built, you can start it this way:
+
+$ rebar3 shell
+
+In the erlang shell:
+1> osmo_dia2gsup:start().

