Produce a full guide to AWS Roles Anywhere and SPIFFE

[strideynet]

As well as showing step by step how to use this utility, the guide should also include SPIFFE and SVID specific advice. The existing AWS Roles Anywhere guides are a little too generic and don't cover very well how to write trust policies designed to authorize specific SPIFFE IDs.

Perhaps publish this guide at https://spiffe.io/docs/ or at least for now store it within this repo as its own markdown file.