Updated documentation related to Keymap and minor fix to return basekeymap

Author: sandeep-vedam

docs/plugins/app.md

@@ -0,0 +1,22 @@
+# App
+
+SDK provides various methods that can be used within the application.
+
+
+## Available Methods
+
+###overRideKeyMap
+
+Over-rides the default keymap with the new keymap.
+
+This can be done using the below snippet :
+
+```js
+this.application.overRideKeyMap(customKeyMap, keepDefaults)
+```
+
+- `customKeyMap` : Keymap to be overidden
+- `keepDefaults`: Flag to overide the the default keymap or not.
+    - By default `keepDefaults` will be false.
+    - If set to `false`, it will override the existing keymap.
+    - If set to `true`, it will be merged with the base keymap along with the exsiting keymap, where it has all the keys which has same values.

docs/plugins/keymap.md

@@ -142,6 +142,7 @@ export default function(App, appData, platformSettings) {
         keymapping[key] = customKeymap[key]
         return keymapping
       }, baseKeymap)
+      return baseKeymap
     }
 
     _setup() {