[INTERNAL] Guidelines: Section for commit message changed
[INTERNAL] Guidelines: Section for commit message changed

- Restructured the section 'Git Guidelines'
- Extended the section with a description for the commit summary

It was discussed that adding some explenation in the commit message
helps the reviewer to understand a change. Thus the problem that should
be fixed should be mentioned in the commit message and it's purpose.

Change-Id: Ia0ad5dd73dc4c4bd7d321c8267f9d976c9fb5428

SAP/openui5,SAP/openui5,cschuff/openui5,cschuff/openui5,SAP/openui5,SQCLabs/openui5,olirogers/openui5,nzamani/openui5,olirogers/openui5,SQCLabs/openui5,nzamani/openui5,SQCLabs/openui5,SAP/openui5,olirogers/openui5,nzamani/openui5,cschuff/openui5