Add doc for pw_build.
Change-Id: I2b8b234adadfffc7c3d533613b15fa54ab3d3d8f
diff --git a/pw_build/build.rst b/pw_build/build.rst
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+++ b/pw_build/build.rst
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+.. _chapter-build:
+
+.. default-domain:: cpp
+
+.. highlight:: sh
+
+--------
+pw_build
+--------
+The build module contains the configuration necessary to build Pigweed using
+either `GN`_/`Ninja`_ or `Bazel`_.
+
+.. _GN: https://gn.googlesource.com/gn/
+.. _Ninja: https://ninja-build.org/
+.. _Bazel: https://bazel.build/
+
+GN / Ninja
+==========
+The common configuration for GN for all modules is in the ``BUILD.gn`` file.
+It contains ``config`` declarations referenced by ``BUILD.gn`` files in other
+modules.
+
+The ``pw_executable.gni`` file contains a wrapper to build a target for the
+globally-defined target type, and ``python_script.gni`` makes it easy to run
+Python scripts as part of build rules.
+
+Bazel
+=====
+The common configuration for Bazel for all modules is in the ``pigweed.bzl``
+file. The built-in Bazel rules ``cc_binary``, ``cc_library``, and ``cc_test``
+are wrapped with ``pw_cc_binary``, ``pw_cc_library``, and ``pw_cc_test``.
+These wrappers add parameters to calls to the compiler and linker.
+
+The ``BUILD`` file is merely a placeholder and currently does nothing.