Add Clock to Help about clock-activity

[EmilyOng]

i would like to take this issue

[quozl]

Thanks!

[EmilyOng]

godiard/help-activity#37

• write a source/clock.rst,
• update source/index.rst,
• update helplink.json,
• make pull requests to https://github.com/godiard/help-activity
• added images

Lesson plans:

• learning to count time
• reading time in different ways
• speaking time
• learning about angles
• finding the geographical north or south

[quozl]

Thanks.