update: 80文字以内にコミットメッセージは収めて欲しいと追記。

Author: tasuren

CONTRIBUTING.md

@@ -3,4 +3,5 @@
 The style of the code is roughly in line with PEP8. Please keep the atmosphere of the code as much as possible.
 ## Commit message
 Put the verb of what you did first, followed by the details.
+And we would like it to contain 80 characters.
 Example: `fix: ループしないように変更。`