Add contributing how-to
Fix chaining of arguments to Transformer yielders
Add non-operator methods for source/transformer/sink connect

There are several advantages to using descriptive methods: They yield
more idiomatic Ruby code, and they are less likely to cause
compatibility issues with existing code (particularly for String, which
already has <= and >= comparison operators). Also, they generally mix
better with other Enumerable methods.

On the other hand, making those same Enumerable methods work as expected
when dealing with partial pipelines requires re-implementing all of the
Enumerable module for Transformer; and they are not lazy by default (see
the section "Connect operators" in the README).

Clearly, more field testing is needed.
Add some more Enumerable methods to Transformer::Lazy
Set RDoc flag in gemspec
Transformer#in_connect: return a lazy enumerator

That makes it one situation less where one has to remember to drop in a
".lazy". Also make Proc#<= return a proper lazy enumerator.

In order to remain compatible with Ruby < 2.0, pull in Charles Oliver
Nutter's lazy_enumerator gem.