Improve readmes

models/README.md

+Prototxt files that define models and solvers.
+
+Three models are defined, with some variations of each to support experiments
+in the paper.
+ - Caffenet (model **S**)
+ - VGG_CNN_M_1024 (model **M**)
+ - VGG16 (model **L**)

output/README.md

+Artifacts generated by the scripts in `tools` are written in this directory.
+
+Trained Fast R-CNN networks are saved under:
+
+```
+output/<experiment directory>/<dataset name>/
+```
+
+Test outputs are saved under:
+
+```
+output/<experiment directory>/<dataset name>/<network snapshot name>/
+```

tools/README.md

+Tools for training, testing, and compressing Fast R-CNN networks.