Add contributing guide for Wave workflow

[BojayL]

Summary

• Adds a root CONTRIBUTING.md covering issue selection, label prefixes, Wave workflow labels, branch naming, PR expectations, and validation guidance.
• Documents the Stellar-native scope and clarifies that WaveFlow has no Drips API dependency.
• Links the contributing guide from the README documentation section.

Fixes #38.

Validation

• git diff --check
• Checked PR-facing metadata and changed files for restricted terms