bazel/README: Add bazel/README

Signed-off-by: Jan Bylicki <[email protected]>

Author: jbylicki

bazel/README.md

@@ -0,0 +1,60 @@
+# OpenROAD-flow-scripts and Bazel integration
+
+`bazel-orfs` is a Bazel package containing the definitions and logic governing the build process of ORFS designs.
+The module uses the `openroad/orfs` docker image to extract the flow scripts with dependencies, builds the Bazel environment around them and defines the methods of calling the ORFS Makefiles with selected designs.
+
+## Run examples
+
+`flow/BUILD.bazel` contains definitions for various flows to serve as examples.
+
+It is recommended to run the utility [Bazelisk](https://github.com/bazelbuild/bazelisk) to manage the version of `bazel` installed on the system.
+Details on installation can be found in the `bazel-orfs` [README](https://github.com/The-OpenROAD-Project/bazel-orfs?tab=readme-ov-file#requirements)
+
+The flow can be ran with the following call structure:
+
+```bash
+bazel build <target_name>_<stage_name>
+```
+
+For example, to run the stage `final`, along with all the dependent stages, call:
+```bash
+bazel build gcd_final
+```
+
+Details on usage and defining of the flows are presented in the Usage section of the `bazel-orfs` [README](https://github.com/The-OpenROAD-Project/bazel-orfs?tab=readme-ov-file#usage)
+
+## Dependency version management
+
+In the flow scipts, the `bazel-orfs` version is defined as
+
+```starlark
+bazel_dep(name = "bazel-orfs")
+git_override(
+    module_name = "bazel-orfs",
+    commit = "<Hash of the default bazel-orfs commit>",
+    remote = "https://github.com/The-OpenROAD-Project/bazel-orfs.git",
+)
+```
+However, as the referenced documentation shows, the git-based dependency can be overridden with a local repository.
+First, remove the `git_override` call entirely and replace it with a `local_path_override` call following this convention:
+
+```starlark
+local_path_override(
+  module_name = "bazel-orfs",
+  path = "/replace/with/path/to/local/orfs/repository"
+)
+```
+
+`bazel-orfs` sets a default version of the docker image used to create the Bazel environment.
+This selection can be overridden by a following snippet inserted below the `bazel-orfs` declaration and override.
+
+```starlark
+orfs = use_extension("@bazel-orfs//:extension.bzl", "orfs_repositories")
+orfs.default(
+    image = "tag-name",
+    sha256 = "the-hash",
+)
+```
+
+Substitute `tag-name` with the tag of the version needed and `the-hash` with the digest corresponding to the selected tag (without the `sha256:` prefix).
+