Clarify the API stability wording

Previously the wording implied that between major versions the API won't change.
But it's pretty clear from the surrounding wording and general semvar concept
that the API _will_ change between major versions... it's only between minor/patch
versions that it won't change.

README.md

@@ -46,7 +46,7 @@ If you'd rather install RuboCop using `bundler`, add a line for it in your `Gemf
 gem 'rubocop', require: false
 ```
 
-RuboCop is stable between major versions, both in terms of API and cop configuration.
+RuboCop is stable between minor versions, both in terms of API and cop configuration.
 We aim to ease the maintenance of RuboCop extensions and the upgrades between RuboCop
 releases. All big changes are reserved for major releases.
 To prevent an unwanted RuboCop update you might want to use a conservative version lock