Updated README.md on points that confused me

I've never used assume role on the CLI before so these additional words seemed to help my understanding.

README.md

@@ -1,4 +1,4 @@
-This is a small utility that makes it easier to use the `aws sts assume-role` command:
+This tool will request and set temporary credentials in your shell environment variables for a given role.
 
 ## Installation
 
@@ -8,31 +8,51 @@ On OS X, the best way to get it is to use homebrew:
 brew install remind101/formulae/assume-role
 ```
 
-If you have a working Go 1.6 environment:
+If you have a working Go 1.6/1.7 environment:
 
 ```bash
 $ go get -u github.com/remind101/assume-role
 ```
 
 ## Configuration
 
-The first step is to setup profiles for the different roles you'd like to assume in `~/.aws/config`. Follow the official AWS docs on how to do this at https://docs.aws.amazon.com/cli/latest/userguide/cli-roles.html.
+Setup a profile for each role you would like to assume in `~/.aws/config`.
 
-**Example**
+**For example, `~/.aws/config`:**
 
 ```ini
+[profile usermgt]
+# IAM user management AWS Account.
+region = us-east-1
+aws_access_key_id = AKIMYFAKEEXAMPLE
+aws_secret_access_key = wJalrXUtnFEMI/K7MDENG/MYxFAKEYEXAMPLEKEY
+
 [profile stage]
+# Stage AWS Account.
 region = us-east-1
 role_arn = arn:aws:iam::1234:role/SuperUser
-mfa_serial = arn:aws:iam::5678:mfa/eric-holmes
-source_profile = default
+usermgtsource_profile = usermgt
 
 [profile prod]
+# Production AWS Account.
 region = us-east-1
 role_arn = arn:aws:iam::9012:role/SuperUser
-source_profile = default
+mfa_serial = arn:aws:iam::5678:mfa/eric-holmes
+source_profile = usermgt
 ```
 
+Reference: https://docs.aws.amazon.com/cli/latest/userguide/cli-roles.html
+
+In this example, we have three AWS Account profiles:
+
+ * usermgt
+ * stage
+ * prod
+
+Each member of the org has their own IAM user and access/secret key in the `usermgt` AWS Account.
+The `stage` and `prod` AWS Accounts have an IAM role named `SuperUser`.
+The `assume-role` tool helps a user authenticate and then assume the privilege of the `SuperUser` role, even across AWS accounts!
+
 ## Usage
 
 Perform an action as the given IAM role:
@@ -41,7 +61,7 @@ Perform an action as the given IAM role:
 $ assume-role stage aws iam get-user
 ```
 
-The command provided after the role will be executed with the `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `AWS_SESSION_TOKEN` environment variables set.
+The `assume-role` tool sets `AWS_ACCESS_KEY_ID`, `AWS_SECRET_ACCESS_KEY` and `AWS_SESSION_TOKEN` environment variables and then executes the command provided.
 
 If the role requires MFA, you will be asked for the token first: