contributing: Add readme to development doc (#2962)

This adds a readme file to doc/development. It contains info which was originally in the main readme, but it misses some info useful for navigating the files under doc/development.

---------

Co-authored-by: Markus Neteler <neteler@osgeo.org>
Co-authored-by: Loïc Bartoletti <lbartoletti@users.noreply.github.com>

doc/development/README.md

@@ -0,0 +1,43 @@
+# Development and Maintenance Documentation
+
+Here is development and maitanance documentation. The API documentation
+is at appropriate places, but here is the information relevant to
+contributing to GRASS GIS and its maintanance.
+
+## How to generate the 'Programmer's Manual'
+
+You can locally generate the [GRASS GIS Programmer's Manual](https://grass.osgeo.org/programming8/).
+
+This needs doxygen (<http://www.doxygen.org>) and optionally
+Graphviz dot (<http://www.research.att.com/sw/tools/graphviz/>).
+
+To build the GRASS programmer's documentation, run
+
+```sh
+make htmldocs
+```
+
+Or to generate documentation as a single html file
+(recommended for simple reading)
+
+```sh
+make htmldocs-single
+```
+
+This takes quite some time. The result is in `lib/html/index.html`
+which refers to further document repositories in
+
+```text
+lib/vector/html/index.html
+lib/db/html/index.html
+lib/gis/html/index.html
+```
+
+The master file is: `./grasslib.dox` where all sub-documents have to
+be linked to.
+
+To generate the documents in PDF format, run
+
+```sh
+make pdfdocs
+```