Update README to document 'token option for skipper-s3

README.md

@@ -109,6 +109,7 @@ req.file('avatar').upload({
   key: 'YOUR_S3_API_KEY',
   secret: 'YOUR_S3_API_SECRET',
   bucket: 'YOUR_S3_BUCKET',
+  token: 'THE_AWS_SESSION_TOKEN_FROM_STS',
   headers: {
     'x-amz-acl': 'YOUR_FILE_PERMISSIONS'
   }
@@ -122,6 +123,7 @@ It exposes the following adapter-specific options:
  key        | ((string))                       | Your AWS "Access Key ID", e.g. `"BZIZIZFFHXR27BFUOZ7"` (_required_)
  secret     | ((string))                       | Your AWS "Secret Access Key", e.g. `"L8ZN3aP1B9qkUgggUnEZ6KzrQJbJxx4EMjNaWy3n"` (_required_)
  bucket     | ((string))                       | The bucket to upload your files into, e.g. `"my_cool_file_uploads"` (_required_)
+ token      | ((string))                       | If you are using [Temporary Security Credentials via AWS Security Token Service](http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp.html) instead of your long-term AWS Access Key, provide the sessionToken in that option, e.g. using `require("aws-sdk").config.credentials.sessionToken`
  endpoint   | ((string))                       | By default all requests will be sent to the global endpoint `s3.amazonaws.com`. But if you want to manually set the endpoint, you can do it with the endpoint option. |
  region     | ((string))                       | The S3 region where the bucket is located, e.g. `"us-west-2"`. Note: If `endpoint` is defined, `region` will be ignored. Defaults to `"us-standard"` |
  tmpdir     | ((string))                       | The path to the directory where buffering multipart requests can rest their heads.  Amazon requires "parts" sent to their multipart upload API to be at least 5MB in size, so this directory is used to queue up chunks of data until a big enough payload has accumulated.  Defaults to `.tmp/s3-upload-part-queue` (resolved from the current working directory of the node process- e.g. your app)