Update readme.txt

changed Title to RatePAY OXID Module for Oxid CE 4.7.x
changed Author to Sören Martius, Aarne Welschlau
changed Version to 3.0.0
changed Link to www.ratepay.com
changed Mail to info@ratepay.com
Merge pull request #6 from bjoernkowarsch/patch-2

Update readme.txt