Update contrib guide

Author: gonzalocasas

CONTRIBUTING.rst

@@ -103,28 +103,40 @@ The best way to send feedback is to file an issue on
 * Explain in detail how it would work.
 * Keep the scope as narrow as possible, to make it easier to implement.
 
-A Note on Architecture for Backend Clients
-------------------------------------------
+Grasshopper components
+----------------------
 
-To maintain consistency from one backend client to another and to promote modularity,
-we make use of several interfaces.  Please reference :ref:`Note on Architecture <architecture>`
-for more details on how to add or amend a backend client.
+Grasshopper user objects need to be built using `COMPAS Github Action componentizer <https://github.com/compas-dev/compas-actions.ghpython_components>`_.
+
+1. Apply your changes to the component source code (``src/compas_fab/ghpython/components``).
+2. Rebuild them:
+
+   .. code-block:: bash
+
+        invoke build-ghuser-components <path_to_ghio.dll>
 
-A Note on GhPython User Components
-----------------------------------
+3. Install them on Rhino/Grasshopper as usual:
 
-The Grasshopper user objects are built with `COMPAS Github Action componentizer <https://github.com/compas-dev/compas-actions.ghpython_components>`_.
+   .. code-block:: bash
 
-If you make changes to the files in ``src/compas_fab/ghpython/components``, the components need to be rebuilt.
-This is done using the ``invoke`` task::
+        python -m compas_rhino.install
 
-    invoke build-ghuser-components <path_to_ghio.dll>
+The install step does not copy them, but creates a symlink to the location in which they are built,
+so after the first installation, it is usually not required to reinstall them, only rebuild them (unless a new component is added).
 
-The components (``.ghuser`` files) are installed using the same command that installs COMPAS on Rhino::
+.. note::
 
-    python -m compas_rhino.install
+    This step requires IronPython 2.7 to be available on the system, ie. `ipy` should be callable from the command line.
+    The path to the GH_IO.dll is platform-specific, on Mac it is under the ``GrasshopperPlugin.rhp`` of the Rhino app
+    and on Windows is in the ``Grasshopper`` folder within the Rhino folder in ``ProgramFiles``.
 
-The installation does not copy them, but creates a symlink to the location in which they are built in this repo.
+
+A Note on Architecture for Backend Clients
+------------------------------------------
+
+To maintain consistency from one backend client to another and to promote modularity,
+we make use of several interfaces.  Please reference :ref:`Note on Architecture <architecture>`
+for more details on how to add or amend a backend client.
 
 Design documents
 ----------------