Add clarification for open source

[kaitj]

Add clarification to the "open source" checklist item (see #39).

[shnizzedy]

I don't know if we can easily include links in the description, but a link to the full OSI definition of "open source" would be more appropriate than "source code is available for viewing", especially when the definition starts with

Open source doesn’t just mean access to the source code.

[shnizzedy]

Alternatively, we could rename the criterion from "open source" to "source-available" or something like that

What is considered "open source" in the context of the checklist?

IIRC, the intention of the "open source" criteria was source availability, so we should make that distinction.

― @mgxd #39 (comment)