Update README

README.md

@@ -56,6 +56,7 @@ Usage: electron-mocha [options] [files]
     -s, --slow <ms>        "slow" test threshold in milliseconds [75]
     -t, --timeout <ms>     set test-case timeout in milliseconds [2000]
     -u, --ui <name>        specify user-interface (bdd|tdd|exports)
+    -w, --watch            watch files for changes
     --check-leaks          check for global variable leaks
     --compilers            use the given module(s) to compile files
     --globals <names>      allow the given comma-delimited global [names]
@@ -64,6 +65,7 @@ Usage: electron-mocha [options] [files]
     --no-timeouts          disables timeouts
     --recursive            include sub directories
     --renderer             run tests in renderer process
+    --watch-extensions     additional extensions to monitor with --watch
 
 ```