Updated documentation

CONTRIBUTING.md

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+# Contributing to operator-libs-linux
+
+## Bugs and pull requests
+
+- Generally, before developing enhancements to this charm, you should consider explaining your use
+  case.
+- If you would like to chat with us about your use-cases or proposed implementation, you can reach
+  us at [Canonical Mattermost public channel](https://chat.charmhub.io/charmhub/channels/charm-dev)
+  or [Discourse](https://discourse.charmhub.io/).
+- All enhancements require review before being merged. Apart from code quality and test coverage,
+  the review will also take into account the resulting user experience for Juju administrators
+  using this charm.
+
+## Setup
+
+A typical setup using [snaps](https://snapcraft.io/) can be found in the [Juju
+docs](https://juju.is/docs/sdk/dev-setup).
+
+## Developing
+
+You can use the environments created by `tox` for development:
+
+```shell
+tox --notest -e unit
+source .tox/unit/bin/activate
+```
+
+### Testing
+
+```shell
+tox -e fmt       # update your code according to linting rules
+tox -e lint      # code style
+tox -e unit      # unit tests
+tox              # runs 'lint' and 'unit' environments
+```
+
+## Build charm
+
+Build the charm in this git repository using:
+
+```shell
+charmcraft pack
+```