Updated documentation for env vars

[pressdarling]

Building on PR #11:

• updated readme.md
• renamed images
• clarified "unsupported currency pairing" error.

I don't speak PHP, so apologies if this doesn't parse but it worked when I tested it. I've made no attempt to change the code based on the comments in #11.

If nothing else, updating the readme should help resolve issues #8 + #10.

[pressdarling]

Sorry if this isn't the right way to go about raising PRs, btw - do what you will with this!