Extend README with directory content.

README.md

@@ -2,5 +2,14 @@
 
 This is a collection of tools, code and examples used for converting user's modelling projects into [ModelCIF format](https://mmcif.wwpdb.org/dictionaries/mmcif_ma.dic/Index/) for upload to [ModelArchive](https://www.modelarchive.org) (MA). The whole repository has grown over time from bulk uploads to MA, we assisted users with. Do not expect everything to work out of the box, old conversion projects may be outdated by now.
 
+## Directories
+
+A short overview of the directories in this repository and what they do.
+
+|Path       |Content                                                         |
+|-----------|----------------------------------------------------------------|
+|projects/  |Collection of model conversions done for various user projects. |
+|validation/|A tool to check the formatting of ModelCIF files.               |
+
 <!--  LocalWords:  modelcif ModelArchive
  -->