Document the need for dev packages since Multiple people asked about it.
Document the need for dev packages since Multiple people asked about it.

8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr,8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr,8thcolor/eurucamp2014-htdsadr