[DOC] port documentation from command line tools from cmic to github

[tvercaut]

The cmic wiki has some interesting informatiomation:
http://cmictig.cs.ucl.ac.uk/wiki/index.php/NiftyReg_documentation

For example the usage description for each executables:
http://cmictig.cs.ucl.ac.uk/wiki/index.php/Reg_transform

It would be helpful to provide the same here. Could be in the wiki
https://github.com/KCL-BMEIS/niftyreg/wiki/executable
or directly as a readme file in the source code:
https://github.com/KCL-BMEIS/niftyreg/tree/master/reg-apps

[mmodat]

Will start this soon as we are almost done with re-instating the CUDA code.