Missing proguard configuration in README.md

[AdamGrzybkowski]

The library in the current state will not work when using proguard.

PhxMessage class member names are used to parse the response from WS.
The solution would be to add @SerializeName annotation, this way there is no need to add any additional proguard rules (apart from the ones for gson and okhhtp).

As for now you have to specify a rule like this
-keepclassmembernames class org.phoenixframework.PhxMessage { *; }

[AdamGrzybkowski]

sory for duplicated issues :( GitHub is really unpredictable today