Fix #518, provide --enforce-all option to check_added_large_files

The --enforce-all option when provided ensures that all files passed
on the command line are checked against the size limit.  Default
behaviour remains unchanged.
diff --git a/README.md b/README.md
index 3552721..a6b62ab 100644
--- a/README.md
+++ b/README.md
@@ -26,8 +26,11 @@
 #### `check-added-large-files`
 Prevent giant files from being committed.
   - Specify what is "too large" with `args: ['--maxkb=123']` (default=500kB).
+  - Limits checked files to those indicated as staged for addition by git.
   - If `git-lfs` is installed, lfs files will be skipped
     (requires `git-lfs>=2.2.1`)
+  - `--enforce-all` - Check all listed files not just those staged for
+    addition.
 
 #### `check-ast`
 Simply check whether files parse as valid python.