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

Author: thepriefy

Diff for: .env_sample

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

Diff for: .gitignore

@@ -323,4 +323,6 @@ $RECYCLE.BIN/
 *.lnk
 
 # Code
-/.vscode
+/.vscode
+
+.env

Diff for: README.md

@@ -103,6 +103,24 @@ For sample implementations, see the [.NET Core Example](https://github.com/sendg
 
 - Please see the [.nuspec file](https://github.com/sendgrid/sendgrid-csharp/tree/master/nuspec).
 
+## 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