Update README with latest Usage

README.md

@@ -24,7 +24,7 @@ You may need to add `~/bin` to your PATH variable if not already done. See [here
 
 # Usage
 ```help
-usage: vimtabdiff.py [-h] [--vim VIM] [--onlydiffs] pathA pathB
+usage: vimtabdiff.py [-h] [--vim VIM] [--onlydiffs] [--skipmissing] pathA pathB
 
 Show diff of files from two directories in vim tabs
 
@@ -36,6 +36,7 @@ options:
   -h, --help     show this help message and exit
   --vim VIM      vim command to run
   --onlydiffs    only open files where there is a diff
+  --skipmissing  skip non-existent files
 ```
 
 ## Relevant vim tips