message: Add documentation on using locally checked out LLVM version.
diff: @@ -9,3 +9,31 @@ dialects and utilities for
  3. TF Lite
  See [MLIR's website](https://mlir.llvm.org) for complete documentation.
  +
  +## Getting started
  +
  +Building dialects and utilities here follow the standard approach using
  +`bazel` as the rest of TensorFlow.
  +
  +### Using local LLVM repo
  +
  +To develop across MLIR core and TensorFlow, it is useful to override the repo
  +to use a local version instead of fetching from head. This can be achieved as
  +below but note, the BUILD files are not automatically generated from or CMake
  +used, so if your change requires a BUILD file change (or you are using a
  +different version of LLVM than set in tensorflow/workspace.bzl's LLVM_COMMIT)
  +then manual BUILD file changes may be required.
  +
  +```sh
  +LLVM_SRC=...
  +
  +# Create basic workspace file
  +echo 'workspace(name = "llvm-project")' > $LLVM_SRC/WORKSPACE
  +# and over the bazel BUILD files.
  +cp third_party/llvm/llvm.autogenerated.BUILD $LLVM_SRC/llvm/BUILD
  +cp third_party/mlir/BUILD $LLVM_SRC/mlir
  +cp third_party/mlir/test.BUILD $LLVM_SRC/mlir/test/BUILD
  +
  +bazel build --override_repository=llvm-project=$LLVM_SRC \
  + -c opt tensorflow/compiler/mlir:tf-opt
  +```
  