Step to create rexray secrets

[kaustavb12]

Hi @BretFisher

Following the steps of deployment in README,
in Step 1, the REXRAY_DO_TOKEN env. variable is set and in Step 5, external secrets rexray_do_token is used during deployment of the rexray stack.

However this secret is not created at any step prior to Step 5.

A command such as echo $REXRAY_DO_TOKEN | docker secret create rexray_do_token - could be included in Step 5, prior to the stack deployment command or a new step 5 can be added can be added and subsequent steps renumbered.

I could open a PR for the same, if you agree.

[BretFisher]

Sure, just add it to step one in a PR, thanks.