common: add three checks to cstyle
common: fix cstyle checking

Make cstyle check benchmark directory and correct cstyle errors
in this directory.
Merge pull request #16 from ldorau/common-fix-cstyle-checking

common: fix cstyle checking
Merge pull request #17 from ldorau/common-add-three-checks-to-cstyle

common: add three checks to cstyle
doc: add instructions for contributing

Fixes: pmem/issues#2