Update forge/core to 2.6.0.Final and correct README

I've followed this [StackOverflow answer](http://stackoverflow.com/a/25677459/2517807) to get Arquillian addon working in JBDS 8.0 with Forge 2.10.1, and stumbled upon couple issues.

Firstly, the command suggested in the README was wrong. Secondly, the build failed because forge_core/2.5.1 is unavailable both from Maven Central and from JBoss Public Repo. Finally, I also had to update the arquillian-setup command instruction bit and make sure that UTF-8 is used by the Maven Resources plugin as well as by the Maven Compiler plugin.

The rest of README is still somewhat wrong, but I couldn't fix it, and honestly don't care much as I prefer Forge GUI, which seems to work OK.