README.md suggestion: add more details

[ninest]

Currently, this is the only information in readme.md:

This is the Git repository for gridsome.org. Feel free to contribute 🥳

I thought it may be useful to add these sections from the from the contribution page:

• Contributing to the docs
• Contributing to the blog
• Submit a Starter

This may make it more clear for those who want to contribute. I'll be happy to create a pull request for this