Create exit command documentation

Partial fix of issue #711
Some parts have been copied from the fish documentation

docs/commands/exit.md

@@ -0,0 +1,17 @@
+# exit
+
+Exits the nu shell. If STATUS is supplied, it will be converted to an integer and used as the exit code. Otherwise, the exit code will be that of the last command executed.
+
+## Examples 
+
+```shell
+> exit
+```
+
+```shell
+> exit 1
+```
+
+```shell
+> exit 0
+```