Add a .env_sample file

[thinkingserious]

Our preference is that users have an environment file when using the SendGrid API, because it is less likely that someone would commit their credentials to github.

Please make a couple changes:

• make a .env_sample file that contains: export SENDGRID_API_KEY=''
• make (or add) a .gitignore file and include .env
• add instructions to the README file about how to copy .env_sample to .env and add the API Key