docs: ✏️ Document changes related to Scala 3

Author: gaeljw

CHANGELOG.md

@@ -11,6 +11,9 @@ See also the [CHANGELOG](https://github.com/cucumber/cucumber-jvm/blob/master/CH
 
 ### Added
 
+- [Scala] Support Scala 3 RC1
+  - Check out the [v7.x upgrade guide](./docs/upgrade_v7.md)
+
 ### Changed
 
 - [Build] Use Github Actions instead of Travis CI

README.md

@@ -25,6 +25,7 @@ The minor version might differ because Cucumber Scala may add Scala-related feat
 
 - [Installation](./docs/install.md)
 - Upgrade notes
+  - [Version 7.x](docs/upgrade_v7.md)
   - [Version 6.x](docs/upgrade_v6.md)
   - [Version 5.x](docs/upgrade_v5.md)
 - Documentation

docs/upgrade_v7.md

@@ -0,0 +1,41 @@
+# Upgrading from 6.x to 7.x
+
+Upgrading from v6 should be straightforward.
+Prior to upgrading to v7.0.0 upgrade to latest v6.x and stop using all deprecated features.
+Some features will log a deprecation warning.
+
+See also:
+- [Cucumber Scala CHANGELOG](../CHANGELOG.md)
+
+## Scala 3 support
+
+This release brings Scala 3 support.
+
+### Syntactic changes
+
+If you use Scala 3, you might need to change slightly some of your glue code:
+- parenthesis are now necessary even around a single-argument step or hook definition
+```scala
+// Won't compile anymore
+Given("Something {}") { str: String =>
+    // ...
+}
+
+// Instead use:
+Given("Something {}") { (str: String) =>
+  // ...
+}
+```
+- hooks must explicitly return `Unit` (most of the time you already had compile errors with such statements in Scala 2.x as well)
+```scala
+Before {
+  // ... // Some code not retuning Unit
+  () // Explicit Unit
+}
+```
+
+### Other changes
+
+The line numbers provided in reports might slightly change
+from start of a step definition to end of a step definition in some cases.
+In case of errors, these line numbers should be more accurate than before.