improve Coding Guidelines

[ipa-srd-jl]

add following points:

• upload ssh key to GitHub
• describe how to use wstool and rosinstall files

[floweisshardt]

@ipa-srd-jl: are you still working on this?

[mgruhler]

@ipa-fmw @ipa-srd-jl is no longer at IPA, but will probably return in August... (so: No, not right now ;-) )