Add doap.xml

[stokito]

The doap.xml contains a list of supported XEPs.
The XMPP.org collects the doap.xml and shows them on https://xmpp.org/software/
E.g. open https://xmpp.org/software/qxmpp/ and click on "Show supported XEPs".
The doap.xml for the page is taken from the library sources https://invent.kde.org/libraries/qxmpp/-/blob/master/doc/doap.xml?ref_type=heads

See:

• XEP-0453.
• http://oss-watch.ac.uk/resources/doap
• https://projects.apache.org/guidelines.html
• https://github.com/ewilderj/doap/blob/master/schema/doap.rdf