Add Rails Icons to README

[eelcoj]

Sidekick Icons was recently added to Rails Icons. Would you be up to add it to the README? Maybe a separate section for it?

[ndri]

Thanks for letting me know! I'm very happy to hear that Sidekickicons are supported in another library. :)

I added Rails instructions to the README. (5dfaaf2)

Am I correct that Heroicons are the default and you need to specify Sidekickicons? Or will it use other iconsets as a fallback when they're not found in the default?

[eelcoj]

Upon installation you can choose any library (and later change which library you want) as a default. Technically you could set Sidekickions as a default. :)