Document criteria for proposing new licenses

[benbalter]

This pull request documents @mlinksva's suggestions for new license criteria over in #315. Specifically:

1. The license must have an SPDX identifier. If you're license isn't register with SPDX, please request that it be added.
2. The license must be listed on one of the following approved lists of licenses:
   • List of OSI approved licenses
   • GNU's list of free licenses (note: the license must be listed in one of three "free" categories)
   • Open Definition's list of conformant licenses (non-code)
3. A GitHub code search must reveal at least 1,000 public repositories using the license

If there's rough consensus that the above is good criteria, I'll adapt the existing tests to confirm the above (and run them against the existing license list). I believe SPDXiness is already checked, as is OSI approval, with a few whitelisted exceptions.

[benbalter]

Believe this is ready for 👀, especially from @mlinksva.

I added tests to enforce the proposed license addition criteria. All our existing licenses meet the criteria.

Excluding GitHub search results (which can't be retrieved programmatically), there are currently 104 potentially "approved" licenses:

aal, afl-1.1, afl-1.2, afl-2.0, afl-2.1, afl-3.0, agpl-3.0, apache-1.1, apache-2.0, apl-1.0, apsl-1.0, apsl-1.1, apsl-1.2, apsl-2.0, artistic-1.0, artistic-1.0-cl8, artistic-1.0-perl, artistic-2.0, bsd-2-clause, bsd-3-clause, bsl-1.0, catosl-1.1, cc-by-4.0, cc-by-sa-4.0, cc0-1.0, cddl-1.0, cecill-2.1, cnri-python, cpal-1.0, cpl-1.0, cua-opl-1.0, ecl-1.0, ecl-2.0, efl-1.0, efl-2.0, entessa, epl-1.0, eudatagrid, eupl-1.1, fair, frameworx-1.0, gpl-2.0, gpl-3.0, hpnd, ijg, imatix, intel, ipa, ipl-1.0, isc, lgpl-2.0, lgpl-2.1, lgpl-3.0, lpl-1.0, lpl-1.02, lppl-1.3c, miros, mit, motosoto, mpl-1.0, mpl-1.1, mpl-2.0, mpl-2.0-no-copyleft-exception, ms-pl, ms-rl, multics, nasa-1.3, naumen, ncsa, ngpl, nokia, nposl-3.0, ntp, oclc-2.0, odbl-1.0, ofl-1.1, ogtsl, osl-1.0, osl-2.0, osl-2.1, osl-3.0, php-3.0, postgresql, python-2.0, qpl-1.0, rpl-1.1, rpl-1.5, rpsl-1.0, rscpl, ruby, simpl-2.0, sissl, sleepycat, spl-1.0, unlicense, upl-1.0, vim, vsl-1.0, w3c, watcom-1.0, wtfpl, xnet, zlib, zpl-2.0

By that same logic, there are 83 potential additions:

aal, afl-1.1, afl-1.2, afl-2.0, afl-2.1, apache-1.1, apl-1.0, apsl-1.0, apsl-1.1, apsl-1.2, apsl-2.0, artistic-1.0, artistic-1.0-cl8, artistic-1.0-perl, bsl-1.0, catosl-1.1, cc-by-4.0, cc-by-sa-4.0, cddl-1.0, cecill-2.1, cnri-python, cpal-1.0, cpl-1.0, cua-opl-1.0, ecl-1.0, ecl-2.0, efl-1.0, efl-2.0, entessa, eudatagrid, eupl-1.1, fair, frameworx-1.0, hpnd, ijg, imatix, intel, ipa, ipl-1.0, lgpl-2.0, lpl-1.0, lpl-1.02, lppl-1.3c, miros, motosoto, mpl-1.0, mpl-1.1, mpl-2.0-no-copyleft-exception, multics, nasa-1.3, naumen, ncsa, ngpl, nokia, nposl-3.0, ntp, oclc-2.0, odbl-1.0, ogtsl, osl-1.0, osl-2.0, osl-2.1, php-3.0, postgresql, python-2.0, qpl-1.0, rpl-1.1, rpl-1.5, rpsl-1.0, rscpl, ruby, simpl-2.0, sissl, sleepycat, spl-1.0, upl-1.0, vim, vsl-1.0, w3c, watcom-1.0, xnet, zlib, zpl-2.0

[waldyrious]

Minor typo: "it's" --> "its". Also, shouldn't "Clear" be capitalized?

[mlinksva]

Looks good overall! See line notes for minor comments.

[benbalter]

Thanks for the feedback @mlinksva. Believe I implemented all your suggestions, but glad to continue to improve if you have any more / I missed any / things change once we start using it. 😄