First version of the contexts document

This would close #732.

Problem is it is part of a larger document, #1225, which is by no
means complete. Also was recently added to #114 as part of the
language docs.

doc/Language/contexts.pod6

@@ -0,0 +1,24 @@
+=begin pod :tag<perl6>
+
+=TITLE Contexts and contextualizers
+
+=SUBTITLE What are contexts and how to get into them.
+
+A context is needed, in many occasions, to interpret the value of a container. In Perl 6, we will use context to coerce the value of a container into some type or class, or decide what to do with it, as in the case of tne sink context.
+
+=head1 Sink
+
+I<Sink> is equivalent to C<void> context, that is, a context in which we throw (down the sink, as it were) the result of an operation or the return value from a block. In general, this context will be invoked in warnings and errors when a statement does not know what to do with that value.
+
+=begin code
+my $sub = -> $a { return $a² };
+$sub; # OUTPUT: «WARNINGS:␤Useless use of $sub in sink context (line 1)␤»
+=end code
+
+You can force that sink context on L<Iterator>s, by using the L<C<sink-all>|/routine/sink-all> method. L<Proc>s can also be L<sunk via the C<sink> method|https://docs.perl6.org/type/Proc#method_sink>, forcing them to raise an exception and not returning anything.
+
+In general, blocks will warn if evaluated in sink context; however, L<gather/take blocks|/language/control#Flow%29_gather_take> are explicitly evaluated in sink context, with values returned explicitly using C<take>.
+
+=end pod
+
+# vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6