Update readme with build instruction

Roughly:
```
git clone git@github.com:google/keytransparency-java.git
cd keytransparency-java
````
Create a `local.properties` file with 
```
ndk.dir=/home/antonio/Android/Sdk/ndk-bundle
sdk.dir=/home/antonio/Android/Sdk
```

```  
go get golang.org/x/mobile/cmd/gomobile
gomobile init -ndk path/to/your/ndk
```

connect your phone and enable usb debugging
`./gradlew installDebug`
