Add CONTRIBUTING.md (#301)

* Add CONTRIBUTING.md

* updates

* updates

* fix

* Update CONTRIBUTING.md

Co-authored-by: Anuraag Agrawal <[email protected]>

Co-authored-by: Anuraag Agrawal <[email protected]>

Author: trask

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+## Contributing
+
+Pull requests for bug fixes are always welcome!
+
+Before submitting new features or changes to current functionality, it is recommended to first
+[open an issue](https://github.com/open-telemetry/opentelemetry-java-contrib/issues/new)
+and discuss your ideas or propose the changes you wish to make.
+
+### Building
+
+In order to build and test this whole repository you need JDK 11+.
+
+#### Snapshot builds
+
+For developers testing code changes before a release is complete, there are
+snapshot builds of the `main` branch. They are available from
+the Sonatype OSS snapshots repository at `https://oss.sonatype.org/content/repositories/snapshots/`
+([browse](https://oss.sonatype.org/content/repositories/snapshots/io/opentelemetry/contrib/))
+
+#### Building from source
+
+Building using Java 11+:
+
+```bash
+java -version
+```
+
+```bash
+./gradlew assemble
+```
+
+### Style guide
+
+See the [Style guide](https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/docs/contributing/style-guideline.md)
+from the opentelemetry-java-instrumentation repository.