Update gitignore (sorry)

electron/node_modules/mkdirp/bin/usage.txt

@@ -0,0 +1,12 @@
+usage: mkdirp [DIR1,DIR2..] {OPTIONS}
+
+  Create each supplied directory including any necessary parent directories that
+  don't yet exist.
+  
+  If the directory already exists, do nothing.
+
+OPTIONS are:
+
+  -m, --mode   If a directory needs to be created, set the mode as an octal
+               permission string.
+