Create release_flow.md

Author: raullenchai

release_flow.md

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+# Release Flow
+This doc lists out the steps for preparing, cutting and rolling out a release of iotex-core. 
+
+## Prepare
+1. Cut release candidate, e.g., v1.2.1_rc1, v1.2.2_rc2
+2. Roll out the candidate to testnet and observe all chain metrics
+3. Put in necessary bug fixes, performance improvements
+4. If necessary repeat steps 1 - 3
+5. Once stablized, revise the tag by removing `rc*`, e.g., v1.2.1_rc5 becomes v1.2.1
+
+## Rollout
+6. Notify the community
+7. Notify all delegates
+8. Notify all exchanges who run nodes