add documentation

doc/changes.rst

@@ -8,9 +8,11 @@ Change Log
 * :command:`desiInstall` uses desihub location of simqso fork (commit e963344_).
 * Allow :command:`desiInstall` to remove permission-locked directories;
   suppress certain :command:`pip` warnings (PR `#185`_).
+* Allow :command:`desiInstall` to compile code in certain branch installs (PR `#188`_).
 
 .. _e963344: https://github.com/desihub/desiutil/commit/e963344cd072255174187d2bd6da72d085745abd
 .. _`#185`: https://github.com/desihub/desiutil/pull/185
+.. _`#188`: https://github.com/desihub/desiutil/pull/188
 
 3.2.5 (2022-01-20)
 ------------------

doc/desiInstall.rst

@@ -322,6 +322,20 @@ not bundled with the code.  The script should download data *directly* to
 with :command:`desiInstall` and unit tests.  Note that here are other, better ways to
 install and manipulate data that is bundled *with* a Python package.
 
+Compile in Branch Installs
+--------------------------
+
+In a few cases (fiberassign_, specex_) code needs to be compiled even when
+installing a branch.  If :command:`desiInstall` detects a branch install *and*
+the script ``etc/product_compile.sh``, :command:`desiInstall` will run this
+script, supplying the Python executable path as a single command-line arguement.
+The script itself is intended to be a thin wrapper on *e.g.*::
+
+    python setup.py build_ext --inplace
+
+.. _fiberassign: https://github.com/desihub/fiberassign
+.. _specex: https://github.com/desihub/specex
+
 Fix Permissions
 ---------------