updated README infos for first-time users (#7218)
updated README infos for first-time users (#7218)

* updated README infos for first-time users

* clarified first two steps for contributing code (text as suggested by bekuno)

* stripped 'step by step' section

* Deleted reference to 'step by step' section

* swapped Eclipse and Android Studio sections

rsudev/c-geo-opensource,cgeo/cgeo,matej116/cgeo,rsudev/c-geo-opensource,cgeo/cgeo,Bananeweizen/cgeo,rsudev/c-geo-opensource,tobiasge/cgeo,matej116/cgeo,auricgoldfinger/cgeo,auricgoldfinger/cgeo,Bananeweizen/cgeo,cgeo/cgeo,S-Bartfast/cgeo,Bananeweizen/cgeo,tobiasge/cgeo,S-Bartfast/cgeo,tobiasge/cgeo,matej116/cgeo,auricgoldfinger/cgeo,S-Bartfast/cgeo,cgeo/cgeo