Update documentation on SystemProperties settings

[victorcampos]

I was using the old documentation found at https://code.google.com/p/t7mp/wiki/Configuration because the new one over http://t7mp.github.io/ doesn't document usage on the part for the plugin.

What I've encountered is that the example over setting SystemProperties under the is wrong. The tags changed from to <MY_KEY>MY_VALUE</MY_KEY> and this part isn't documented anywhere, I've found the answer digging in the source code and with this answer on GitHub: #50

Could you please update the documentation or provide me a way to edit it?

Cheers