Add documentation for the 'svgio' module

The module 'svgio' is used to read SVG. This is
the list of topics discussed in the documentation:

* Introduction
* How does it work?
* Known Bugs
* Dependencies
* Related Software
* References

Change-Id: If42e9c2e10145c12cd5d74019b36bc5d78dd5bb9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/124178
Tested-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>
Reviewed-by: Ilmari Lauhakangas <ilmari.lauhakangas@libreoffice.org>

svgio/README.md

@@ -1,3 +1,36 @@
 # SVG Reader
 
-It contains `svgio/source/svgreader` which is used for "Insert -> Picture -> From File".
+## Introduction
+The **svgio** module is used to read **SVG** (Scalable Vector Graphics[1])
+files. It is an XML based format for vector graphics.
+
+This module contains `svgio/source/svgreader` which is used for embedding an
+SVG file with "Insert -> Picture -> From File".
+
+SVG is an open standard provided by the World Wide Web Consortium (W3C).
+
+[1] [Scalable Vector Graphics](https://en.wikipedia.org/wiki/Scalable\_Vector\_Graphics)
+
+## How does it work?
+`svgio` module uses sax for reading xml and turns it into `drawinglayer` primitives.
+The rendering is done via `drawinglayer` primitives. For more information, you should
+refer to [drawinglayer](../drawinglayer) documentation.
+
+## Known Bugs
+Known remaining bugs for this module are gathered here:
+
+* [Bug 88278 - [META] SVG import image filter (all modules)](https://bugs.documentfoundation.org/show\_bug.cgi?id=88278)
+
+## Dependencies
+Direct dependencies for **emfio** are [**drawinglayer**](../drawinglayer) and
+[**sax**](../sax).
+
+## Related Software
+* [librsvg](https://en.wikipedia.org/wiki/Librsvg)
+* [SVG++](http://svgpp.org/)
+
+## References
+Documentation for the SVG format is available on the W3C website:
+
+* [SVG page at W3C](https://www.w3.org/Graphics/SVG/)
+* [SVG primer](https://www.w3.org/Graphics/SVG/IG/resources/svgprimer.html)