add the link definition for the phased rollout

docs/README.mdx

@@ -28,3 +28,5 @@ Please feel free to file issues in the Soroban repos, or, if you prefer, to rais
 ## Why it's called Soroban
 
 The Japanese soroban is the smallest and simplest abacus. Its design is intentionally compact and minimalist, focused on doing the essentials — and doing them well. It set the global standard for abacuses and is still in practical use today. The name aligns with the goals of this platform because we, too, want to build something that focuses on doing the essentials well, that sets a global standard, and that proves useful far into the future.
+
+[phased rollout]: https://stellar.org/blog/developers/the-stellar-network-s-phased-rollout-of-smart-contracts-the-road-to-mainnet