add symbol usage to docs

[motine]

A minor documentation update on how to use the Observable module with procs/lambdas. For me this was not obvious at first glance, so I hope this would help others...

[motine]

Hey @hsbt, what do you think?

[motine]

@tenderlove, @jeremyevans, @mrkn, @luislavena, do you know who is active in this repo?

[hsbt]

I only handle the release management. I'm not sure this documentation patch is good or bad.

[motine]

thanks @hsbt for the quick reply. can you point me to someone who I can approach?

[marcandre]

Great example, as there wasn't any with a specific method to call. Thanks!