Document how to build ORC integration

cpp/README.md

@@ -145,6 +145,13 @@ The CUDA toolchain used to build the library can be customized by using the
 This library is still in Alpha stages, and subject to API changes without
 deprecation warnings.
 
+### Building Apache ORC integration (optional)
+
+The optional arrow reader for the Apache ORC format (found in the
+`arrow::adapters::orc` namespace) can be built by passing `-DARROW_ORC=on`.
+This is currently not supported on windows. Note that this functionality is
+still in Alpha stages, and subject to API changes without deprecation warnings.
+
 ### API documentation
 
 To generate the (html) API documentation, run the following command in the apidoc

python/doc/source/development.rst

@@ -175,6 +175,9 @@ Now build and install the Arrow C++ libraries:
 If you don't want to build and install the Plasma in-memory object store,
 you can omit the ``-DARROW_PLASMA=on`` flag.
 
+To add support for the experimental Apache ORC integration, include
+``-DARROW_ORC=on`` in these flags.
+
 Now, optionally build and install the Apache Parquet libraries in your
 toolchain:
 
@@ -205,6 +208,9 @@ Now, build pyarrow:
 If you did not build parquet-cpp, you can omit ``--with-parquet`` and if
 you did not build with plasma, you can omit ``--with-plasma``.
 
+If you built with the experimental Apache ORC integration, include
+``--with-orc`` in these flags.
+
 You should be able to run the unit tests with:
 
 .. code-block:: shell