Add mapping page

Not important but mark to here first.

Needs to add:
 - How to mapping (Tricky)
   - Tricky way to create basic beatmap with lyric.
   - Tricky way to create note.
   - Tricky way to create translate
- Copyright issue
  - What is copyright
  - How to ask permission from youtube
- Customize video format
  - What video is suitable for karaoke, which part should be removed or improved.


