Add CONTRIBUTING.md (#2555)

Author: trask

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing
+
+Welcome to OpenTelemetry community repository!
+
+Before you start - see OpenTelemetry general [contributing](guides/contributor/README.md)
+requirements and recommendations.
+
+## Updating SIG information on the README.md
+
+The SIG tables on the [README.md](README.md) page are generated from [`sigs.yml`](sigs.yml).
+
+To regenerate those tables after updating any `sigs.yml` content, run:
+
+```bash
+make table-generation
+```
+
+## Updating the assets.md page
+
+The [`assets.md`](assets.md) page has an auto-generated table of contents.
+To regenerate the table of contents after updating any headings on that page, run:
+
+```bash
+make markdown-toc
+```