Add CONTRIBUTING (#104)

Adds initial contributing guide.

CONTRIBUTING.md

@@ -0,0 +1,120 @@
+# Contributing
+
+We welcome your contributions to this project!
+
+Please read the [OpenTelemetry Contributor Guide][otel-contributor-guide]
+for general information on how to contribute including signing the Contributor License Agreement, the Code of Conduct, and Community Expectations.
+
+## Before you begin
+
+### Specifications / Guidelines
+
+As with other OpenTelemetry clients, opentelemetry-ruby follows the
+[opentelemetry-specification][otel-specification] and the
+[library guidelines][otel-lib-guidelines].
+
+### Focus on Capabilities, Not Structure Compliance
+
+OpenTelemetry is an evolving specification, one where the desires and
+use cases are clear, but the method to satisfy those uses cases are not.
+
+As such, Contributions should provide functionality and behavior that
+conforms to the specification, but the interface and structure are flexible.
+
+It is preferable to have contributions follow the idioms of the language
+rather than conform to specific API names or argument patterns in the spec.
+
+For a deeper discussion, see: https://github.com/open-telemetry/opentelemetry-specification/issues/165
+
+## Getting started
+
+Everyone is welcome to contribute code via GitHub Pull Requests (PRs).
+
+### Fork the repo
+
+Fork the project on GitHub by clicking the `Fork` button at the top of the
+repository and clone your fork locally:
+
+```sh
+git clone git@github.com:YOUR_GITHUB_NAME/opentelemetry-ruby.git
+```
+
+or
+```sh
+git clone https://github.com/YOUR_GITHUB_NAME/opentelemetry-ruby.git
+```
+
+It can be helpful to add the `open-telemetry/opentelemetry-ruby` repo as a
+remote so you can track changes (we're adding as `upstream` here):
+
+```sh
+git remote add upstream git@github.com:open-telemetry/opentelemetry-ruby.git
+```
+
+or
+
+```sh
+git remote add upstream https://github.com/open-telemetry/opentelemetry-ruby.git
+```
+
+For more detailed information on this workflow read the
+[GitHub Workflow][otel-github-workflow].
+
+### Run the tests
+
+_Setting up a running Ruby environment is outside the scope of this document._
+
+This repository contains two Ruby gems:
+
+  * `opentelemetry-api`: located at `api/opentelemetry-api.gemspec`
+  * `opentelemetry-sdk`: located at `sdk/opentelemetry-sdk.gemspec`
+
+Each of these gems has its configuration and tests.
+
+For example, to test the `api` you would:
+
+  1. Change directory to `api`
+  2. Run the tests with ```rake test```
+
+### Make your modifications
+
+Always work in a branch from your fork:
+
+```sh
+git checkout -b my-feature-branch
+```
+
+### Create a Pull Request
+
+You'll need to create a Pull Request once you've finished your work.
+The [Kubernetes GitHub Workflow][kube-github-workflow-pr] document has
+a significant section on PRs.
+
+Open the PR against the `open-telemetry/opentelemetry-ruby` repository.
+
+Please put `[WIP]` in the title, or create it as a [`Draft`][github-draft] PR
+if the PR is not ready for review.
+
+#### Sign the Contributor License Agreement (CLA)
+
+All PRs are automatically checked for a signed CLA. Your first PR fails this
+check if you haven't signed the [CNCF CLA][cncf-cla].
+
+The failed check displays a link to `details` which walks you through the
+process. Don't worry it's painless!
+
+### Review and feedback
+
+PRs require a review from one or more of the [code owners](CODEOWNERS) before
+merge. You'll probably get some feedback from these fine folks which helps to
+make the project that much better. Respond to the feedback and work with your
+reviewer(s) to resolve any issues.
+
+
+[cncf-cla]: https://identity.linuxfoundation.org/projects/cncf
+[github-draft]: https://github.blog/2019-02-14-introducing-draft-pull-requests/
+[kube-github-workflow-pr]: https://github.com/kubernetes/community/blob/master/contributors/guide/github-workflow.md#7-create-a-pull-request
+[otel-contributor-guide]: https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md
+[otel-github-workflow]: https://github.com/open-telemetry/community/blob/master/CONTRIBUTING.md#github-workflow
+[otel-lib-guidelines]: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/library-guidelines.md
+[otel-specification]: https://github.com/open-telemetry/opentelemetry-specification