Update README.md

most importantly adding the preference for rebasing local commits over merging - but also filled it out a bit more with some relevant bits from the frostwire-android project README to make some points more explicit

* copied over some bits from the "coding standards" and "contribution guidelines" sections of the frostwire-android project

* removed "git pull" command from the "Getting the source" section - this followed immediatey after a "git clone" command which inherently pulls in the latest HEAD

* removed the paragraph mentioning the "eclipse.formatter.xml" file as it has apparently just been deleted from the repo

* minor cosmetic changes

regarding the windows instruction for setting environment variables in the "Build Requirements" section - (just as a note - this section was not altered) - im pretty sure that the java installer sets JAVA_HOME and adds it to the PATH automatically - CLASSPATH im not sure tho but the process is not at all similar unless one builds under something like cygwin or minGW - at least as i remember on XP setting environment variables persistently must be done via the gui "system properties"