add contributing guidelines

CONTRIBUTING.md

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+# Contribution Guide
+
+:tada: Thanks for taking the time to contribute! :tada:
+
+The following is a set of guidelines for contributing to this repository, which is hosted in the [Codecov Organization](https://github.com/codecov) on GitHub. 
+
+## What does this repo do?
+
+This repo is a GitHub Action, meaning it integrates with the GitHub Actions CI/CD pipeline. It's mean to take formatted reports with code coverage stats and upload them to codecov.io. What's essentially happening in the background is that Actions is spinning up a Linux Docker container with the contents of this repository. Inside that container, we then call a shell scipt that runs Codecov's Bash uploader. 
+
+## Improvements
+
+Here's a list of things that would extend the functionality of this Action:
+
+* **Specify file path**:     Currently, there's no way to specify a file path to a coverage report. This would have to be a user input that gets passed through to the Docker container
+* **Codecov YAML location**: No way to indicate a path to the `codecov.yml` file. Obtain from user input.
+* **Upload flags**: Cannot currently specify any flags. Obtain from user input.
+
+This is primarily an effort in bash scripting and getting the information above from a user through to make a correct curl call