Mention third_party repo is mozilla-only, provide alternatives

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Author: Manishearth

README.md

@@ -16,12 +16,19 @@ git clone [email protected]:MozillaReality/FirefoxReality.git
 
 *Clone the third_party repo*
 
-If you're developing for the Oculus, Snapdragon VR, or VIVE, you need to clone the repo with third party SDK files.
+If you're developing for the Oculus, Snapdragon VR, or VIVE, you need to clone the repo with third party SDK files. 
 
 ```bash
 git clone [email protected]:MozillaReality/FirefoxReality-android-third-party.git third_party
 ```
 
+This repo is only available to Mozilla employees. If you have access to the relevant SDK but not this repo, you can manually place them here:
+
+ - `third_party/ovr_mobile/` for Oculus (should contain a `VrApi` folder)
+ - `third_party/svr/` for Snapdragon (should contain a `libs` folder, among other things)
+ - `third_party/wavesdk/` for Vive (should contain a `build` folder, among other things)
+
+
 *Fetch Git submodules.*
 
 You may need to set up [two-factor authentication](https://blog.github.com/2013-09-03-two-factor-authentication/#how-does-it-work-for-command-line-git) for the command line.