Docs: Intro to Yosys source section

docs/source/yosys_internals/extending_yosys/extensions.rst

@@ -8,6 +8,8 @@ Writing extensions
 
 .. todo:: update to use :file:`/code_examples/extensions/test*.log`
 
+.. todo:: should this be moved to yosys_source section?
+
 This chapter contains some bits and pieces of information about programming
 yosys extensions. Don't be afraid to ask questions on the YosysHQ Slack.
 

docs/source/yosys_source/index.rst

@@ -1,6 +1,10 @@
 Yosys source details
 --------------------
 
+This section goes into additional detail on the Yosys source code and git
+repository.  This information is not needed for simply using Yosys, but may be
+of interest for developers looking to customise Yosys builds.
+
 .. toctree::
 
    build_verific