Add GenePred release notes (minor doc fix)
NGB now supports gene data in [genePred](http://genome.ucsc.edu/FAQ/FAQformat#format9) file format.
Two variants of the format are supported: `Gene Predictions` (10 columns) and `Gene Predictions (Extended)` (16 columns).
-Files with extensions `.genepred` and `.gp` (case-insensitive) are supported. Use CLI to register genePred files:
+Files with extensions `.genepred`, `.genePred` and `.gp` are supported. Use CLI to register genePred files:
```
# Add genePred file as a gene file to GRCh38 reference
$ ngb ag GRCh38 /opt/ngb/data/homo_sapience.genePred
