Add link to where people can find me to README

Prawn is sort of a unique project, in that it is based on the work of so many,
but was started (and has been at times sustained by) people in the community
trusting me and supporting my efforts financially.

With that in mind, I think some folks have been attracted to the project
for precisely that reason. Because I'm continuing that sort of work over at
Practicing Ruby, I'd like to in a very subtle way let people know how
to find me!

(Personally would be fine with adding links to any maintainer's Twitter account,
but I'll leave that up to other maintainers to decide.)

README.md

@@ -148,9 +148,10 @@ If you contribute to Prawn, you will retain your own copyright but must agree to
 
 ## History
 
-Prawn was originally developed by Gregory Brown, under the auspices of the Ruby
-Mendicant Project, a grassroots initiative in which the Ruby community
-collectively provided funding so that Gregory could take several months off from
+Prawn was originally developed by [Gregory Brown](http://twitter.com/practicingruby), 
+under the auspices of the Ruby Mendicant Project, a grassroots initiative in which 
+the Ruby community collectively provided funding so that Gregory could take 
+several months off from
 work to focus on this project.
 
 Over the last several years, we've received code contributions from dozens of