doc: 增加 preminor 例子

README.md

@@ -23,9 +23,10 @@ npm version [<newversion> | major | minor | patch | premajor | preminor | prepat
 
 npm version preminor --preid alpha --no-git-tag-version
 npm version preminor --preid beta --no-git-tag-version
-
+1.2.0 -> 1.3.0-beta.0
 npm version premajor --no-git-tag-version
 1.0.0 -> 2.0.0-0
+
 npm version prerelease --no-git-tag-version
 2.0.0-0 -> 2.0.0-1 -> 2.0.0-2 ..n -> 2.0.0-n
 npm version patch --no-git-tag-version