Added example file, updated .gitignore and README.

Author: dhsrocha

.gitignore

@@ -14,3 +14,6 @@ CSharpHTTPClient/packages/
 CSharpHTTPClient/CSharpHTTPClient.sln.VisualState.xml
 *.PublicKey
 *.pfx
+
+# Environment files
+.env/*.*

Example/.env_sample

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

README.md

@@ -86,6 +86,8 @@ Console.WriteLine(response.Body.ReadAsStringAsync().Result);
 Console.WriteLine(response.Headers.ToString());
 ```
 
+You can use the ```.env_example``` file located at "examples" folder. Just copy it to a ```.env``` folder ou can put on your project's root.
+
 <a name="usage"></a>
 # Usage