docs(clients-matrix.md): Introducing the library features matrix.
As support for etcd features is very uneven between different clients,
a feature matrix can help application developers to understand what a
client library can offer them, and also help client libraries
developers to understand what they should work on. I assessed the
features of all client libraries by looking at their master branch on
github, to the best of my knowledge.
Merge pull request #441 from lavagetto/matrix

docs(clients-matrix.md): Introducing the library features matrix.