Add README.md for BCR mirror

Change-Id: I4e6c3af1fafde920ee1a73846effbdd13ff43017

Author: TimotheusBachinger

bazel/registry/modules/README.md

@@ -0,0 +1,26 @@
+# BCR Mirror
+This is an attempt to mirror the bazel central registry on our nexus server.
+
+# How To
+In case an upstream resource (most likely github) is unavailable and the corresponding package is not already part of this mirror here, do the following:
+* create the folder structure under `bazel/registry/modules`
+  * `{module_name}/{version}/source.json`
+  * you can also easily copy it from [bazel-central-registry](https://github.com/bazelbuild/bazel-central-registry/tree/main/modules)
+  * e.g. in order to get `alsa_lib` in version `1.2.0`, copy `alsa_lib/1.2.9/source.json` including the folders to `bazel/registry/modules`
+* afterwards upload the file from `url` to nexus's `upstream-archives`
+* add a field to the `source.json` which points to the nexus mirror:
+```
+"mirror_urls": {"https://artifacts.lan.tribe29.com/repository/upstream-archives/{THE_FILE_NAME_FROM_url}"}
+```
+
+# Test the mirror
+* clean the repository cache:
+```
+rm -rf ~/.cache/bazel/_bazel_$(whoami)/cache/repos/v1
+```
+* clean the bazel cache:
+```
+bazel clean --expunge
+```
+* test the mirror by blocking the upstream url (e.g. by introducing a typo in the url) and do a `bazel build` which would pull that depend
+This should result in using the nexus url for downloading.