Add README file to src/ directory

src/README.md

+This directory contains the source code of CoPaR, except tests and benchmarks.
+
+### Subdirectory overview
+
+- `main` contains code for the main `copar` executable
+- `prism-converter` contains a converter for files from the PRISM benchmark
+  suite into CoPaR's own as well as various other file formats.
+- `random-dfa` contains a generator for random deterministic finite automata
+- `random-wta` contains a generator for random weighted tree automata
+- Everything else belongs to the Haskell library that contains most of CoPaR's
+  functionality.