Recommend license attribute in gemspec

[bf4]

Per discussion in rubygems/guides#67

1. Adds a license attribute to the example gemspec
2. Adds categories for Required and Optional gemspec attributes sections
3. Add 'Recommended' category for license/licenses within the Optional section
4. Add documentation to license, re: github chooser and legal importance
5. Add recommendation to warning (and test)

@drbrain I built the spec guide and it looked fine. RUBYGEMS_DIR=$HOME/repos/forked/rubygems rake spec_guide --trace

[drbrain]

Overall this looks good!

I would comment directly on the lines, but I am on the bus and desktop view typing is painfully slow.

Can you remove the empty :category: lines? They aren't needed because category only applies to methods and attributes.

Does the updated WARNING text fit on one line with 80 columns? I would prefer the URL to stand alone so it's easier to cut and paste.

[bf4]

Per @drbrain, remove

[bf4]

@drbrain updated, also updated the related PR in rubygems/guides#67 (not sure what your usual process is)

[bf4]

Great! It looks like the spec page in the guides should be manually regenerated?

[drbrain]

That will happen automatically with the next release. I'm not sure if the changes to specification are cosmetic only, so I'd like to wait.

[bf4]

@drbrain Sorry, forgot about the github feature of linking issues... will fix

[drbrain]

No problem! If github sends emails for these I don't recall if I noticed