Fix: random segmentation fault during detach using physics mutex

[zabealbe]

As already shown here if a detach is issued during a physics engine update a segmentation fault will be generated, resulting in Gazebo crashing.

I would like to share with you my own solution to the problem which I had the opportunity to extensively test and validate during the development of this project. The solution is simple, efficient and reliable and only consists in using the physics mutex accessible through physics->GetPhysicsUpdateMutex();

[awesomebytes]

I love the simplicity of the fix!

Cold you please remove the readme changes so i can click on merging straight away?

Thank you very much!!

[zabealbe]

Yep, Done