📝 Add powerSaveBlocker APIs doc.

docs/api/power-save-blocker.md

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+# power-save-blocker
+
+The `power-save-blocker` module is used to block the system from entering
+low-power(sleep) mode.
+
+An example is:
+
+```javascript
+var powerSaveBlocker = require('power-save-blocker');
+
+powerSaveBlocker.start(powerSaveBlocker.PREVENT_DISPLAY_SLEEP);
+console.log(powerSaveBlocker.IsStarted());
+```
+
+## powerSaveBlocker.start(type)
+
+* type - Power save blocker type
+  * powerSaveBlocker.PREVENT_APP_SUSPENSION - Prevent the application from being
+    suspended. On some platforms, apps may be suspended when they are not visible
+    to the user.  This type of block requests that the app continue to run in that
+    case,and on all platforms prevents the system from sleeping.
+    Example use cases: downloading a file, playing audio.
+  * powerSaveBlocker.PREVENT_DISPLAY_SLEEP - Prevent the display from going to sleep.
+    This also has the side effect of preventing the system from sleeping, but
+    does not necessarily prevent the app from being suspended on some platforms
+    if the user hides it.
+    Example use case: playing video.
+
+Starts the power save blocker preventing the system entering lower-power mode.
+
+## powerSaveBlocker.isStarted()
+
+Returns whether the `powerSaveBlocker` starts.
+
+## powerSaveBlocker.stop()
+
+Stops blocking the system from entering low-power mode.