serverless-dotenv-plugin 
Preload Environment Variables Into Serverless
Use this plugin if you have variables stored in a .env file that you want loaded into your serverless yaml config. This will allow you to reference them as ${env:VAR_NAME} inside your config and it will load them into your lambdas.
Install and Setup
First, install the plugin:
> npm i -D serverless-dotenv-plugin
Next, add the plugin to your serverless config file:
service: myService
plugins:
- serverless-dotenv-plugin
...
Now, just like you would using dotenv in any other JS application, create your .env file in the root of your app:
DYANMODB_TABLE=myTable
AWS_REGION=us-west-1
AUTH0_CLIENT_ID=abc12345
AUTH0_CLIENT_SECRET=12345xyz
Plugin options
By default, the dotenv package will look for your .env file in the same folder where you run the command, but this can be customized by setting the path option. Also, be default, ALL env vars found in your file will be injected into your lambda functions. If you do not want all of them to be injected into your lambda functions, you can whitelist them with the include option.
custom:
dotenv:
path: ../../.env
include:
- AUTH0_CLIENT_ID
- AUTH0_CLIENT_SECRET
The path option can also be used in combination with serverless variables.
custom:
dotenv:
path: .env-${self:provider.stage}
Usage
Once loaded, you can now access the vars using the standard method for accessing ENV vars in serverless:
...
provider:
name: aws
runtime: nodejs6.10
stage: dev
region: ${env:AWS_REGION}
...
Lambda Environment Variables
Again, remember that when you deploy your service, the plugin with inject these environment vars into any lambda functions you have and will therefore allow you to reference them as process.env.AUTH0_CLIENT_ID (Nodejs example).