chore(docs): Add profile choosing note in readme

closes #77

Author: MikaAK

README.md

@@ -17,7 +17,12 @@ Note: This plugin needs NodeJS > 0.12.0
 ### Usage Instructions
 > I notice a lot of people are setting the `directory` option when the files are part of their build. Please don't set   `directory` if you're uploading your build. Using the `directory` option reads the files after compilation to upload instead of from the build process.
 
-> You can also use a [credentials file](https://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs) from AWS.
+> You can also use a [credentials file](https://blogs.aws.amazon.com/security/post/Tx3D6U6WSFGOK2H/A-New-and-Standardized-Way-to-Manage-Credentials-in-the-AWS-SDKs) from AWS. To set the profile set your s3 options to the following:
+```
+s3Options: {
+  credentials: new AWS.SharedIniFileCredentials({profile: 'PROFILE_NAME'})
+}
+```
 
 > s3UploadOptions default to `ACL: 'public-read'` so you may need to override if you have other needs. See [#28](https://github.com/MikaAK/s3-plugin-webpack/issues/28#issuecomment-309171024)