bazel: add bazel build hack tip

Signed-off-by: Øyvind Harboe <[email protected]>

Author: oharboe

docs/user/BuildLocally.md

@@ -12,6 +12,21 @@ cd OpenROAD-flow-scripts
 sudo ./setup.sh
 ```
 
+## Using Bazel to build OpenROAD and run the ORFS flow
+
+Long story short: OpenROAD will eventually switch to using Bazel for downloading dependencies and building OpenROAD for all the reasons that the DependencyInstaller.sh and cmake are hard to support and brittle across platforms.
+
+Currently the simplest way to build OpenROAD and run ORFS is to run one test, which will download all OpenROAD dependencies and build OpenROAD in the exec configuration:
+
+``` shell
+cd tools/OpenROAD
+bazelisk test src/drt/...
+cd ../flow
+make OPENROAD_EXE=$(pwd)/../tools/OpenROAD/bazel-out/k8-opt-exec-ST-*/bin/openroad
+```
+
+Bazel could similarly be used to download and make available pre-built binaries for tools such as Yosys, eqy and KLayout.
+
 ## Build
 
 ``` shell