Document alternative type declaration syntax
Document alternative type declaration syntax

In the introduction to syntax for declaring data types, document the ability
to declare types via constructor signatures in addition to the Haskell-like
syntax.

This syntax seems to be documented nowhere (not even in the book), and
is just used without explanation when introducing dependent types like
`Fin` that require it. From personal experience, this is *extremely* confusing
to newcomers to Idris.

(I'm not sure if this is the best place to document it, but it needs to be
somewhere where it will be easily found, and this is what comes up for
a Google search for "idris data types".)
kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev,kojiromike/Idris-dev