Suggest adding proguard rules to readme

[mirrajabi]

Getting the following error while using proguard.

Warning:com.github.florent37.expectanim.core.Expectations: can't find referenced class com.github.florent37.expectanim.core.Expectations$GravityIntDef

It can be fixed by adding the below lines in proguard-rules.pro file

-keep class com.github.florent37.expectanim.*{ *; }
-dontwarn com.github.florent37.expectanim.**

[florent37]

thanls 👍