update readme

Author: lorenmh

README.md

@@ -5,9 +5,25 @@ The data is in a JSON format. Each JSON file contains an array of objects,
 where each object has two fields:
 
 * `image`: a 784-element array of numbers [0,255], representing pixel values
-  for each 28x28 handwritten image. 0 means background (white) and 255 means
+  for each 28x28 handwritten image. `0` means background (white) and `255` means
   foreground (black).
 * `label`: the label for this character
 
 There are two JSON files; one is training data and the other is test data.
 These two sets were provided from the original MNIST dataset.
+
+## File sizes
+
+    70M   mnist_handwritten_test.json
+    421M  mnist_handwritten_train.json
+    2.8M  mnist_handwritten_test.json.gz
+    17M   mnist_handwritten_train.json.gz
+
+**I strongly suggest you download the gzipped files, as the non-gzipped files
+are very large**
+
+To decompress:
+
+    gunzip *.gz
+
+If you clone you will need [git-lfs](https://git-lfs.github.com/).