Add a guide to migrate from godeps

[resouer]

It would be great to add a guide of migrating from godeps to vndr

cc @LK4D4

[LK4D4]

@resouer I used awk script for that which I lost :(
Will appreciate help with that.

[resouer]

@LK4D4 Thanks! But we decide not to change our dep tool for now ... I will leave this issue open to see if anyone can help.