add docs for start command (#1816)

docs/commands/start.md

@@ -0,0 +1,26 @@
+# start
+
+Opens each file/directory/URL using the default application.
+
+Syntax: `start ...args{flags}`
+
+### Parameters
+
+* `args`: a list of space-separated files to open
+
+### Flags
+
+    -a --application <string>
+      Specifies the application used for opening the files/directories/urls
+
+## Example
+
+Open `index.html` in the system's default browser (cross platform):
+```shell
+> start index.html
+```
+
+Open `index.html` in Firefox (specific path for OSX):
+```shell
+start index.html -a /Applications/Firefox.app
+```