Add README.md

README.md

+# Readme for wr-cores
+
+wr-cores is the repository containing HDL for White Rabbit cores.
+You also need the wrpc-sw repository if you want to build the software from
+scratch.
+
+See https://ohwr.org/project/wr-cores/-/wikis/wrpc-core
+
+## Development
+
+Starting from 2025, the development branch is the branch master.
+
+## Releases
+
+Old releases have been tagged.
+
+For wrpc v5.0, tag is wrpc-v5.0 
+https://ohwr.org/project/wr-cores/-/wikis/wrpc-release-v5.0
+
+For wrpc v4.2, tag is wrpc-v4.2 
+https://ohwr.org/project/wr-cores/-/wikis/Wrpc-release-v42
+