Documented Constants

docs/docs/constants.md

+# Constants
+
+## statusBarHeight
+```js
+const statusBarHeight = Navigation.constants().statusBarHeight;
+```
+
+## topBarHeight
+```js
+const topBarHeight = Navigation.constants().topBarHeight;
+```
+
+## bottomTabsHeight
+```js
+const bottomTabsHeight = Navigation.constants().bottomTabsHeight;
+```