Update README.md

Added some brief documentation

Author: danesparza

README.md

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 # plex2slack-lambda [![CircleCI](https://circleci.com/gh/danesparza/plex2slack-lambda.svg?style=shield)](https://circleci.com/gh/danesparza/plex2slack-lambda)
 AWS Lambda handler for sending plex webhook messages to a slack channel
+
+## Quick start
+### AWS Lambda setup
+- Download the latest release and setup a new Go based lambda function in AWS.  The handler name is `plex2slack-lambda`
+- Add the environment variable `PLEX2SLACK_WEBHOOK_URL` to your lambda handler.  Set this to your Slack incoming webhook url.  It should look something like *https://hooks.slack.com/services/SOMETHING/SOMEID/SOMETHINGELSEHERE*
+
+### AWS API Gateway setup
+- Create a new AWS API Gateway API.  
+- In the 'Settings' section for the AWS API Gateway for the new API, make sure to add `multipart/form-data` under *Binary Media Types*
+- Create a resource (call it something like 'plex') and create a `POST` method on the resource.  Wire up the lambda handler on the method.  Make sure to select *Use Lambda Proxy integration*
+- Deploy the API