Add license internal link (#10143)
Add license internal link (#10143)

Closes #10135 
Added license to `What's in this document section` linking it to corresponding section within the `README` file.
ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,0x80/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby