Update the README to add a section about how to Customize the Toolbar Buttons

[radazzouz]

Came up in Z#14836

---

Details

Updated the README to add a section about how to Customize the Toolbar Buttons.

Acceptance Criteria

• No API change. Does not impact Android or UWP.
• When approved, right before merging, rebase with master and increment the package version in package.json, package-lock.json, and samples/Catalog/package.json (see example commit: 1bf805f).
• Create a new release (and tag) with the new package version (see https://github.com/PSPDFKit/react-native/releases).

[steipete]

lgtm!

[nickwinder]

LGTM

[steviki]

Nitpick (feel free to ignore):Maybe we could add a sentence on what the second parameter is (the animated flag) and what it can be used for.

[radazzouz]

I'd rather keep this paragraph short. The animated and the view mode parameters are documented in the API here:

react-native/index.js

Lines 362 to 381 in 04aee96

	/**
	* Set the right bar button items for the specified view mode.
	* Note: The same button item cannot be added to both the left and right bar button items simultaneously.
	*
	* @param items The list of bar button items. See the full list of button items here: https://pspdfkit.com/api/ios/Classes/PSPDFViewController.html#/Toolbar%20button%20items
	* @param viewMode The optional view mode. Can be 'document', 'thumbnails', 'documentEditor' or `null`. If `null` is passed, bar button items for all view modes are set.
	* @param animated The animated flag.
	*
	* @platform ios
	*/
	setRightBarButtonItems = function(items, viewMode, animated) {
	if (Platform.OS === "ios") {
	NativeModules.PSPDFKitViewManager.setRightBarButtonItems(
	items,
	viewMode,
	animated,
	findNodeHandle(this.refs.pdfView)
	);
	}
	};

[radazzouz]

I will not bump the version numbers on this PR. I'll do it in #255 for both PRs.