devguide: update readme

Use it to explain how to go about the sequence diagram images
(generation, updating, what is mscgen etc).

Also remove portion that referred to Sphinx builds, as these don't make
sense now.

doc/userguide/devguide/README.md

@@ -1,14 +1,9 @@
 # Suricata Developer Guide
 
-This directory contains the Suricata Developer Guide. The
-[Sphinx Document Generator](http://sphinx-doc.org) is used to build the
-documentation. For a primer os reStructuredText see the
-[reStructuredText Primer](http://sphinx-doc.org/rest.html).
+This directory contains the Suricata Developer's Guide. It is built as part of the Suricata Userguide.
 
-## Verifying Changes
+The Sequence Diagrams seen in the Transactions documentation are generated with Mscgen. Mscgen is a small program to parse Message Sequence Charts that can be represented as text and can then converted to image.
 
-There are a number of output formats to choose from when making the source documentation locally (e.g. html, pdf, man).
+If you need to update the diagrams, please edit the ``.msc`` files present in the diagrams directory (extending/app-layer/diagrams). Once those have been changed, in the ``scripts`` directory (in the main Suricata dir) there's a scrip that will generate images for all files: ``generate-images.sh`` (you'll have to install Mscgen for that to work).
 
-The documentation source can be built with `make -f Makefile.sphinx html`. Substitute the 'html' word for desired output format.
-
-There are different application dependencies based on the output desired.
+More info about Mscgen can be found at: https://www.mcternan.me.uk/mscgen/