add command for updating third_party (Igalia#893)

* add command for updating `third_party`

compilation errors are thrown if the developer's checkout of the third-party repo (in their `third_party` directory) is not up to date. this command fixes that.

* fix URL in README for `third_party`

* tweak git command for `third_party` (per @bluemarvin's feedback)

notice the `git checkout origin/master` in case `third_party` was checked out to a different branch/sha

Author: cvan

README.md

@@ -41,6 +41,11 @@ This repo is only available to Mozilla employees. If you have access to the rele
  - `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)
 
+The [repo in `third_party`](https://github.com/MozillaReality/FirefoxReality-android-third-party) can be updated like so:
+
+```bash
+pushd third_party && git fetch && git rebase origin/master && git checkout origin/master && popd
+```
 
 *Fetch Git submodules.*