Add a .env_sample file, update gitignore, update README.md

.env_sample

@@ -0,0 +1 @@
+export SENDGRID_API_KEY=''

.gitignore

@@ -10,4 +10,5 @@
 # Output of the go coverage tool, specifically when used with LiteIDE
 *.out
 
-.DS_Store
+.DS_Store
+.env

README.md

@@ -38,6 +38,25 @@ All updates to this library is documented in our [CHANGELOG](https://github.com/
 ```bash
 go get github.com/sendgrid/smtpapi-go
 ```
+
+## Setup Environment Variables
+
+### Initial Setup
+
+```bash
+cp .env_sample .env
+```
+
+### Environment Variable
+
+Update the development environment with your [SENDGRID_API_KEY](https://app.sendgrid.com/settings/api_keys), for example:
+
+```bash
+echo "export SENDGRID_API_KEY='YOUR_API_KEY'" > sendgrid.env
+echo "sendgrid.env" >> .gitignore
+source ./sendgrid.env
+```
+
 <a name="quick-start"></a>
 # Quick Start