Add slack #jp-mentoring channel

This commit adds slack #jp-mentoring channel.
This channel is used for disscussion and Q&A about mentoring
in Japanese.

communication/slack-config/channels.yaml

@@ -113,6 +113,7 @@ channels:
   - name: jenkins-x-user
   - name: jp-dev
   - name: jp-events
+  - name: jp-mentoring
   - name: jp-users
   - name: jp-users-novice
   - name: jsonnet