Add expectation excerpt in contributing.md for instrumentations (#482)

Author: lzchen

CONTRIBUTING.md

@@ -160,3 +160,8 @@ For a deeper discussion, see: https://github.com/open-telemetry/opentelemetry-sp
   as specified with the [napolean
   extension](http://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html#google-vs-numpy)
   extension in [Sphinx](http://www.sphinx-doc.org/en/master/index.html).
+
+## Expectations from contributors
+
+OpenTelemetry is an open source community, and as such, greatly encourages contributions from anyone interested in the project. With that being said, there is a certain level of expectation from contributors even after a pull request is merged, specifically pertaining to instrumentations. The OpenTelemetry Python community expects contributors to maintain a level of support and interest in the instrumentations they contribute. This is to ensure that the instrumentation does not become stale and still functions the way the original contributor intended. Some instrumentations also pertain to libraries that the current memebers of the community are not so familiar with, so it is necessary to rely on the expertise of the original contributing parties.
+