docs(*): add per-feature supported environment info. (#33)

Author: sphippen

ENVIRONMENTS.md

@@ -0,0 +1,20 @@
+SUPPORTED ENVIRONMENTS
+======================
+
+The Firebase Web SDK is _officially supported_ in the following environments:
+
+|                   | Auth                                                           | Database | Messaging | Storage              |
+| ----------------- | :------------------------------------------------------------: | :------: | :-------: | :------------------: |
+| IE9               |  ✓                                                             |    ✓     |     ✗     |    ✗                 |
+| IE10-11           |  ✓                                                             |    ✓     |     ✗     |    ✓                 |
+| Edge              |  ✓                                                             |    ✓     |     ✗     |    ✓                 |
+| Firefox           |  ✓                                                             |    ✓     |     ✓     |    ✓                 |
+| Chrome            |  ✓                                                             |    ✓     |     ✓     |    ✓                 |
+| Chrome on iOS     |  ✓                                                             |    ✓     |     ✗     |    ✓                 |
+| Safari            |  ✓                                                             |    ✓     |     ✗     |    ✓                 |
+| React Native      |  ✓ (minus phone auth and popup/redirect OAuth operations)      |    ✓     |     ✗     |    ✓ (minus uploads) |
+| Node.js           |  ✓ (minus phone auth and popup/redirect OAuth operations)      |    ✓     |     ✗     |    ✗                 |
+| Chrome Extensions |  ✓ (minus phone auth, only supports popup OAuth operations)    |    ✓     |     ✗     |    ✓                 |
+| Cordova           |  ✓ (minus phone auth, only supports redirect OAuth operations) |    ✓     |     ✗     |    ✓                 |
+
+Features are not guaranteed to work outside officially supported environments.

README.public.md

@@ -190,7 +190,7 @@ Instead, you can use the
 
 If you use the
 [Closure Compiler](https://developers.google.com/closure/compiler/) or
-compatable IDE, you can find API definitions for all the Firebase JavaScript API
+compatible IDE, you can find API definitions for all the Firebase JavaScript API
 in the included `/externs` directory in this package:
 
 ```
@@ -206,3 +206,8 @@ externs/
 
 The Firebase changelog can be found at
 [firebase.google.com](https://firebase.google.com/support/release-notes/js).
+
+## Browser/environment compatibility
+
+See [ENVIRONMENTS.md](ENVIRONMENTS.md) to see which browsers/environments are
+supported for each feature.