improve documentation re Matchers

[playfulThinking]

Unless I missed it, the docs don't mention that to use the provided matchers, you have to

import ShouldI.Matchers.Context

When I don't, I get:

** (CompileError) test/server_test.exs:15: undefined function should_not_have_key/1
expanding macro: ShouldI.with/2
test/server_test.exs:5: ServerTests (module)
(elixir) lib/code.ex:363: Code.require_file/2
(elixir) lib/kernel/parallel_require.ex:50: anonymous fn/4 in Kernel.ParallelRequire.spawn_requires/5

Thanks for the great library!

Mike