prepare for future PRs, mention small_string revert

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changes
 
+## NEXT RELEASE
+
+- ...
+
 ## 0.20
 
 - add several new `bytes` combinators:
@@ -10,7 +14,7 @@
 - add new `string` combinators and aliases:
   - `{QCheck,QCheck2}.Gen.{string_small,string_small_of}`
   - `QCheck.{string_small,string_small_of,string_of,string_printable,string_printable_of_size,string_small_printable,string_numeral,string_numeral_of_size}`
-- `QCheck2.small_string` character generator argument is no more optional
+- (`QCheck2.small_string` character generator argument is no more optional - reverted again due to backwards incompatibility)
 - add an optional argument with conservative default to `Shrink.string`
 - fix shrinkers in `QCheck.{printable_string,printable_string_of_size,small_printable_string,numeral_string,numeral_string_of_size}` [#257](https://github.com/c-cube/qcheck/issues/257)
 - add `QCheck2.Gen.set_shrink` to modify the generator's shrinker