NEW ISSUE: Missing XSD for contributions

Target: SCA Assembly Model Specification, section 10.2.2 (SCA
Contribution Metadata Document)

Description:

The assembly spec defines that a contribution is made to the SCA domain
by files present in the META-INF directory (sca-contribution.xml ,
sca-contribution-generated.xml). Unfortunately the appendix (A. XML
Schemas) does not define a schema for these XML files. Thus there is no
possibility for any tooling / runtime implementation to validate & parse
officially such XML file.

Proposal:
Add the attached XSD file to the appendix with the XML schemas