Add element for software homepage

[Zachary Dunn]

This is similar to #31. I propose a new optional field software.homepage, that contains a url to a projects public facing homepage. Trackers such as https://fediverse.network and https://fediverse.party provide a link to homepages, but you can see from https://fediverse.network/hubzilla and https://fediverse.party/en/hubzilla/ that it is a manual process that could lead to incorrect links. (The homepage listed for hubzilla is different between the two trackers)

Current implementors are using metadata.software.homepage:

• p3k
• Write Freely
• PixelFed