Add install instructions in Readme.md

[daxid]

Hi,

Installation is not trivial.
Instructions should be put in Readme.md

For instance :

• clone repository into any location
• cd /path/to/mailspring-keybase and then npm install
• start Mailspring and add plugin - you should see a success notification
• close Mailspring and edit config.json and remove keybase from disabled plugins list (config.json can be found in different places depending on your OS. Search for the Mailspring folder in your filesystem)
• start Mailspring, key management can be found in preferences

Best regards

David

[guizmoau]

following those instructions but I can't get any features
on debian stretch

[NgoHuy]

I merged the pull request as #9
Please testing it on your environment