docs: related to #36 : added the .env_sample and instructions in the readme to configure it

.env_sample

@@ -0,0 +1 @@
+SENDGRID_API_KEY='<INSERT YOUR API KEY HERE>';

.gitignore

@@ -1,3 +1,6 @@
+# environment variables
+.env
+
 # Logs
 logs
 *.log

README.md

@@ -31,6 +31,16 @@ All updates to this library are documented in our [CHANGELOG](https://github.com
 
 - Node.js version 0.10, 0.12 or 4
 
+You need to have a API key to use the app. To configure the API key in the environment variables follow these steps.
+ - create .env file in the root directory of the project.
+ - Copy the contents of .env_sample file in to the .env file
+ - Once you have obtained the API key, find the environment variable SENDGRID_API_KEY and assign your API key as it's value.
+ As an example it should look like this
+ ```
+ SENDGRID_API_KEY='Ef34Gyt&dfPFC34G';
+ ```
+
+
 ## Install Package
 
 ```bash

lib/client.js

@@ -1,5 +1,6 @@
 'use strict'
 
+require('dotenv').config();
 var https = require('https')
 var http = require('http')
 var queryString = require('querystring')

package.json

@@ -18,7 +18,9 @@
   "engines": {
     "node": ">= 0.10"
   },
-  "dependencies": {},
+  "dependencies": {
+    "dotenv": "^4.0.0"
+  },
   "devDependencies": {
     "chai": "^3.5.0",
     "eslint": "^2.7.0",