Makes the TP values into properties for use in target-platform pom

* Disable the attachment of the teiid-designer.target platform since its
  identical to the target platform configuration now downloaded via mvn

* The generation of the teiid-designer.target can remain since devs can use
  this for specifying their TP in eclipse IDE

* Adds a README for developers to understand how to create a .target file
  for use with eclipse IDE