Added README.md

README.md

+Usage
+=====
+
+```bash
+./wbdbg_container.sh <IMAGE_NAME>
+```
+
+Example:
+```bash
+./wbdbg_container.sh i10git.cs.fau.de:5005/walberla/buildenvs/gcc:7
+```
+
+waLBerla will be cloned for you at `/walberla` and an initial CMake cache is created at `/build`.
+
+Requirements
+============
+
+You have to be a member of the `docker` group and able to run docker commands as regular user.
+If you use images from a private regsitry (e.g. `i10git.cs.fau.de:5005`) make sure to login first using `docker login`.
+For example:
+```
+docker login i10git.cs.fau.de:5005
+```
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