feat(doc): Add CONTRIBUTING.md (#375)

* feat(doc): Add CONTRIBUTING.md

* feat(ci): Ensure that PRs are signed off

Author: Ferenc-

.github/workflows/pr_commits_signed_off.yml

@@ -0,0 +1,16 @@
+name: Find signed commits
+on:
+  pull_request_target:
+    branches:
+      - master # or the name of your main branch
+jobs:
+  check-sign-off:
+    name: Write comment if unsigned commits found
+    env:
+      FORCE_COLOR: 1
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: live627/check-pr-signoff-action@v1
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}

CONTRIBUTING.md

@@ -0,0 +1,88 @@
+## Contributing In General
+Our project welcomes external contributions. If you have an itch, please feel
+free to scratch it.
+
+To contribute code or documentation, please submit a [pull request](https://github.com/instana/python-sensor/pulls).
+
+A good way to familiarize yourself with the codebase and contribution process is
+to look for and tackle low-hanging fruit in the [issue tracker](https://github.com/instana/python-sensor/issues).
+<!--Before embarking on a more ambitious contribution, please quickly [get in touch](#communication) with us.-->
+
+**Note: We appreciate your effort, and want to avoid a situation where a contribution
+requires extensive rework (by you or by us), sits in backlog for a long time, or
+cannot be accepted at all!**
+
+### Proposing new features
+
+If you would like to implement a new feature, please [raise an issue](https://github.com/instana/python-sensor/issues)
+before sending a pull request so the feature can be discussed. This is to avoid
+you wasting your valuable time working on a feature that the project developers
+are not interested in accepting into the code base.
+
+### Fixing bugs
+
+If you would like to fix a bug, please [raise an issue](https://github.com/instana/python-sensor/issues) before sending a
+pull request so it can be tracked.
+
+<!--
+### Merge approval
+
+The project maintainers use LGTM (Looks Good To Me) in comments on the code
+review to indicate acceptance. A change requires LGTMs from two of the
+maintainers of each component affected.
+
+For a list of the maintainers, see the [MAINTAINERS.md](MAINTAINERS.md) page.
+-->
+
+## Legal
+
+Each source file must include a license header for the MIT
+License. Using the SPDX format is the simplest approach.
+e.g.
+
+```
+/*
+Copyright <holder> All Rights Reserved.
+
+SPDX-License-Identifier: MIT
+*/
+```
+
+We have tried to make it as easy as possible to make contributions. This
+applies to how we handle the legal aspects of contribution. We use the
+same approach - the [Developer's Certificate of Origin 1.1 (DCO)](https://github.com/hyperledger/fabric/blob/master/docs/source/DCO1.1.txt) - that the Linux® Kernel [community](https://elinux.org/Developer_Certificate_Of_Origin)
+uses to manage code contributions.
+
+We simply ask that when submitting a patch for review, the developer
+must include a sign-off statement in the commit message.
+
+Here is an example Signed-off-by line, which indicates that the
+submitter accepts the DCO:
+
+```
+Signed-off-by: John Doe <[email protected]>
+```
+
+You can include this automatically when you commit a change to your
+local git repository using the following command:
+
+```
+git commit -s
+```
+
+<!--
+## Communication
+**FIXME** Please feel free to connect with us on our [Slack channel](link).
+
+## Setup
+**FIXME** Please add any special setup instructions for your project to help the developer
+become productive quickly.
+
+## Testing
+**FIXME** Please provide information that helps the developer test any changes they make
+before submitting.
+
+## Coding style guidelines
+**FIXME** Optional, but recommended: please share any specific style guidelines you might
+have for your project.
+-->