Update Gonk README with bootstrap information

The bootstrap binaries have been approved for public use, so we do not need to request that everyone rebuild B2G in order to try out the B2S build.

r? @Manishearth

ports/gonk/README.md

@@ -4,9 +4,19 @@
 
 Follow the steps [here](https://github.com/servo/servo/wiki/Building-for-Android) for setting up the Android NDK
 and toolchain.
+
+## Bootstrap B2G
+
+We have a bootstrap of the require B2G prebuilt binaries, assuming that you are targeting a Flame device.
+
+```
+curl https://servo-rust.s3.amazonaws.com/B2G/B2G.tgz
+tar zxvf B2G.tgz
+```
+
 ## Build B2G
 
-Note: this will take a long time and will take around 20GB of space
+If you are not using a Flame or just would like to have a local build of the binaries, you can also build B2G locally. Note: this will take a long time and will take around 20GB of space
 
 Disable the screen timeout on the device, and connect to wifi. Also connect it to your computer
 with remote debugging enabled.
@@ -32,8 +42,7 @@ Then run
 
 ## Build B2S
 
-Either set the corresponding `b2g` key in `.servobuild` to the path to the B2G clone (along with), or set the `$GONKDIR`
-environment variable.
+Either set the corresponding `b2g` key in `.servobuild` to the path to the B2G bootstrap or clone (along with), or set the `$GONKDIR` environment variable.
 
 Do the same for the `ndk` and `toolchain` keys (`$ANDROID_NDK` and `$ANDROID_TOOLCHAIN` respectively)