add note about spack help --spec

[cmelone]

closes #191

[cmelone]

I'm not actually sure how useful this is. There's no reference to spack spec in the basics tutorial and the help text itself wouldn't necessarily be useful to a user trying to learn the spec syntax

[tgamblin]

I think it's good -- I believe @adamjstewart added this. I actually think it would be good to put spack spec in the basics tutorial.

[alecbcs]

I'd vote for spack spec -h instead of spack help --spec but otherwise this looks good to me. Thanks for fixing the note directives and putting the codeblock in the note.

[cmelone]

I mistyped, the output should be fixed now. Follow-on PRs will add spack spec to the basics tutorial and auto-gen spack help --spec output