Add initial 4_statefulTesting.md
Fix some wording and typo.
Apply PR's comments.
Fix typo.
Apply PR's comment again. https://github.com/yukitos/FsCheckDocsJP/commit/3b728c313090a5e0bf882c0622392db147d43c35#docs-4_statefultesting-md-P45
Merge pull request #9 from yukitos/master

Add 4_statefulTesting of Japanese version.