[scip-to-glean facebookincubator#8] Add README.md for scip.proto

Author: simonmar

glean/lang/scip/indexer/scip_to_glean/cli/proto/README.md

@@ -0,0 +1,12 @@
+# SCIP Protocol Buffer Definition
+
+`scip.proto` is from the [SCIP project](https://github.com/sourcegraph/scip).
+
+To update it, copy the latest version from upstream:
+
+```
+curl -o scip.proto https://raw.githubusercontent.com/sourcegraph/scip/main/scip.proto
+```
+
+Rust bindings are generated automatically by `build.rs` using
+`protobuf-codegen` during `cargo build`.