Add contributing guide

CONTRIBUTING.md

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+# Contribution guide
+
+Thank you for using net-ldap. If you'd like to help, keep these guidelines in
+mind.
+
+## Submitting a New Issue
+
+If you find a bug, or would like to propose an idea, file a [new issue][issues].
+Include as many details as possible:
+
+- Ruby and Rubygem version
+- LDAP server version
+- Queries, connection information, any other input
+- output or error messages
+
+## Sending a Pull Request
+
+[Pull requests][pr] are always welcome!
+
+Check out [the project's issues list][issues] for ideas on what could be improved.
+
+Before sending, please add tests and ensure the test suite passes.
+
+To run the full suite:
+
+  `bundle exec rake`
+
+To run a specific test file:
+
+  `bundle exec ruby test/test_ldap.rb`
+
+To run a specific test:
+
+  `bundle exec ruby test/test_ldap.rb -n test_instrument_bind`
+
+Pull requests will trigger automatic continuous integration builds on
+[TravisCI][travis]. To run integration tests locally, see the `test/support`
+folder.
+
+## Styleguide
+
+```ruby
+# 1.9+ style hashes
+{key: "value"}
+
+# Multi-line arguments with `\`
+MyClass.new \
+  foo: 'bar',
+  baz: 'garply'
+```
+
+[issues]: https://github.com/jch/html-pipeline/issues
+[pr]: https://help.github.com/articles/using-pull-requests
+[travis]: https://travis-ci.org/ruby-ldap/ruby-net-ldap/