[doc] Document release roadmap + stability goals

README.md

@@ -31,6 +31,18 @@ See [here][GitHub releases] for earlier releases.
 
 Note that Tempel is [not intended](../../wiki/3-Faq#can-i-decrypt-tempel-data-with-other-tools) for interop with other cryptographic tools/APIs.
 
+## Roadmap
+
+Tempel has a fixed scope, and is **fully complete**. I'm happy with its design and implementation, and believe it meets all its objectives in its current form. I'm not anticipating significant changes.
+
+Still, given the sensitivity of the problem domain, I plan to approach Tempel's official stable release as a phased rollout to allow time for user feedback before locking things down:
+
+- Jan 2024 - beta
+- Mar 2024 - release candidate
+- May 2024 - stable
+
+The stable release will be marked as "**done / mature**". From that point, the library is expected to need+see minimal maintance.
+
 ## Disclaimer
 
 **Important**: while Tempel has been written and tested with care, the nature of the problem domain inevitably means that bugs and/or misuse can be **especially harmful and/or easy to make**.