Turns out we didn't solve the problem with the previous commit, as
"10.7" still needs to be in Homebrew's `MacOS::Version::SYMBOLS` hash in
order to be recognised.

To work around this, depend on Lion if Homebrew provides it, otherwise
display a message in the Caveats section about the minimum OS version.

I got this error recently:

    $ brew tap teddywing/formulae
    Updating Homebrew...
    ...
    ==> Tapping teddywing/formulae
    Cloning into '/usr/local/Homebrew/Library/Taps/teddywing/homebrew-formulae'...
    ...
    Error: Invalid formula: /usr/local/Homebrew/Library/Taps/teddywing/homebrew-formulae/HomebrewFormula/dome-key.rb
    dome-key: unknown version :lion
    Error: Cannot tap teddywing/formulae: invalid syntax in tap!

Turns out `:lion` was removed as a Mac OS X version symbol from
Homebrew:
https://github.com/homebrew/brew/commit/1aa8ad09e212cd9cd2f3ae55f899ed4ef03ae4d0

Use a version string instead.

Instruct users to reload mappings if `dome-key` is running. This could
happen if the install commands were copy-pasted from the website:

    $ brew install teddywing/DomeKey/dome-key
    $ brew services start teddywing/DomeKey/dome-key

At this point, `dome-key` would already have been started, and the new
mappings wouldn't be loaded into the running process.

Also indent the shell commands to differentiate them from the text.

Only significant change is the addition of the license to the man page.