chore: Add CONTRIBUTING.md (#230)

Add a CONTRIBUTING.md file with basic instructions for contributors to Go SDK

Author: mjc1283

CONTRIBUTING.md

@@ -0,0 +1,49 @@
+# Contributing to the Optimizely Go SDK
+
+We welcome contributions and feedback! All contributors must sign our [Contributor License Agreement (CLA)](https://docs.google.com/a/optimizely.com/forms/d/e/1FAIpQLSf9cbouWptIpMgukAKZZOIAhafvjFCV8hS00XJLWQnWDFtwtA/viewform) to be eligible to contribute. Please read the [README](README.md) to set up your development environment, then read the guidelines below for information on submitting your code.
+
+## Development process
+
+1. Fork the repository and create your branch from master.
+2. Please follow the [commit message guidelines](https://github.com/angular/angular/blob/master/CONTRIBUTING.md#-commit-message-guidelines) for each commit message. 
+3. Make sure to add tests!
+4. Ensure lint check and tests are passing: `make lint`, `make test`
+5. Push a branch with your changes to GitHub.
+6. Open a PR from your fork into the master branch of the original repo
+9. A repository maintainer will review your pull request and, if all goes well, squash and merge it!
+
+## Pull request acceptance criteria
+
+**All code must have test coverage.** Changes in functionality should have accompanying unit tests. Bug fixes should have accompanying regression tests.
+
+## License
+
+All contributions are under the CLA mentioned above.
+
+For this project, Optimizely uses the Apache 2.0 license, and so asks that by contributing your code, you agree to license your contribution under the terms of the [Apache License v2.0](http://www.apache.org/licenses/LICENSE-2.0).
+
+Your contributions should also include the following header:
+```
+/****************************************************************************
+ * Copyright YEAR, Optimizely, Inc. and contributors                        *
+ *                                                                          *
+ * Licensed under the Apache License, Version 2.0 (the "License");          *
+ * you may not use this file except in compliance with the License.         *
+ * You may obtain a copy of the License at                                  *
+ *                                                                          *
+ *    http://www.apache.org/licenses/LICENSE-2.0                            *
+ *                                                                          *
+ * Unless required by applicable law or agreed to in writing, software      *
+ * distributed under the License is distributed on an "AS IS" BASIS,        *
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. *
+ * See the License for the specific language governing permissions and      *
+ * limitations under the License.                                           *
+ ***************************************************************************/
+
+```
+
+The YEAR above should be the year of the contribution. If work on the file has been done over multiple years, list each year in the section above. Example: Optimizely writes the file and releases it in 2018. No changes are made in 2019. Change made in 2020. YEAR should be “2018, 2020”.
+
+## Contact
+
+If you have questions, please contact [email protected].