Update CONTRIBUTING.md with Slack references
Update CONTRIBUTING.md with Slack references

Slack has replaced Gitter as the channel of communication between the
maintainers of NodeGit.
jmurzy/nodegit,jmurzy/nodegit,nodegit/nodegit,nodegit/nodegit,jmurzy/nodegit,jmurzy/nodegit,nodegit/nodegit,nodegit/nodegit,nodegit/nodegit,jmurzy/nodegit