Address Goa issue #2107 (#90)
Address Goa issue #2107 (#90)

* Address Goa issue #2107

* Add instructions to install protoc

to Getting Started guide.

goadesign-jp/goa.design,goadesign-jp/goa.design,goadesign/goa.design,goadesign-jp/goa.design,goadesign/goa.design,goadesign/goa.design