Fix #2600. Link back to exact docs folder in repo.

src/blenderbim/docs/devs/writing_docs.rst

@@ -1,6 +1,18 @@
 Writing docs
 ============
 
+A great way to contribute without writing code is to help writing
+documentation. Please reach out before contributing as the software is still in
+an alpha state and portions may not be worth documenting as it changes too
+frequently.
+
+Writing technical documentation
+-------------------------------
+
+All documentation is written in ReStructured Text and is available in the
+`BlenderBIM Add-on docs directory
+<https://github.com/IfcOpenShell/IfcOpenShell/tree/v0.7.0/src/blenderbim/docs>`_.
+
 The following colours and annotation styles should be used for annotating
 images. All stroke widths are 3px with a corner radius of 3px.  Horizontal
 underlines are 5px with a corner radius of 2px. The dark green is **39b54a** and