Add Kaigi on Rails videos

[okuramasafumi]

I followed the instruction at docs/contributing.md, but there are some issues left:

• language fields are missing, most of them are Japanese but not all
• some date fields are missing
• Somehow two keynotes are wrongly places
• Some speaker names are now in Japanese, not possible to make it English automatically
• slides_url fields are missing, although we know it as organizers

We have our own YAML file to manage some of these data, so is it a good idea to use it as a data source?