Update README about `PYTHON_PATCH_PATH`.

plugins/python-build/README.md

@@ -104,6 +104,10 @@ process.
 * `PYTHON_CONFIGURE_OPTS` and `PYTHON_MAKE_OPTS` allow you to specify
   configure and make options for buildling CPython. These variables will
   be passed to Python only, not any dependent packages (e.g. libyaml).
+* `PYTHON_PATCH_PATH` allows you to specify a directory that contains
+  the patches for building CPython. All patches should be created
+  as same strip number (default `-p0`). `PYTHON_PATCH_OPTS` allows
+  you to override the options for `patch`.
 
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