grpc/test/core/end2end/README

Each fixture (under fixtures/) is paired with each test (under tests/) and
forms a complete end-to-end test.

To add a new test or fixture:
- add the code to the relevant directory
- update gen_build_json.py to reflect the change
- regenerate projects
// MOE:begin_strip
- update net/grpc/c/BUILD to reflect the change
// MOE:end_strip