Add proguard rules to README and Setup wiki

[jasonwyatt]

Research and provide proguard rules for SRML in README.md and the Setup wiki page.

[usernotnull]

Sorry I don't have enough experience to help you out with this but I can tell you this, if it helps. After using minify enabled and resource shrinking enabled, the library doesn't need any special proguard rules EXCEPT for custom tags created by user, in that case he needs to add:

-keep class com.package_of_custom_srml_tags.** { *; }

[jasonwyatt]

That's great news, thanks for looking into it! If you want to submit a PR for changes to README.md with a section about Proguard, I'd love to review+merge it.