some minor typo and grammar fixes
Merge pull request #2545 from ysim/contributing-readme-changes

some minor typo and grammar fixes
projects/*/linux: remove option to disable GPU powermanagement on remaining projects

Signed-off-by: Stephan Raue <stephan@openelec.tv>
Merge branch 'master' of github.com:OpenELEC/OpenELEC.tv
Merge branch 'master' of github.com:OpenELEC/OpenELEC.tv into openelec-3.2
Merge branch 'openelec-3.2' of github.com:OpenELEC/OpenELEC.tv into openelec-3.2