Add a migration guide

src/site/markdown/migration.md

@@ -2,6 +2,12 @@
 
 This document will help you migrate your code to the latest _acme4j_ version.
 
+## Migration to Version 2.1
+
+- This version adds [JSR 305](https://jcp.org/en/jsr/detail?id=305) annotations. If you use a null-safe language like Kotlin, or tools like SpotBugs, your code may fail to compile because of detected possible null pointer dereferences and unclosed streams. These are potential bugs that need to be resolved.
+
+- In _acme4j_'s `JSON` class, all `as...()` getters now expect a value to be present. For optional values, use `JSON.Value.optional()` or `JSON.Value.map()`. This class is rarely used outside of _acme4j_ itself, so you usually won't need to change anything.
+
 ## Migration to Version 2.0
 
 _acme4j_ 2.0 fully supports the ACMEv2 protocol. Sadly, the ACMEv2 protocol is a major change.