Update README.md

I believe on line 83, Customer should be Customers because api.Customer is undefined.
Minor typo in docs

I found this minor typo in https://docs.balancedpayments.com/current/api.html?language=php#create-a-new-debit under the hold_uri description.

Not sure if this is where it's getting the description pulled from, but I couldn't find it.
Merge pull request #17 from misbahkhan/patch-1

Update README.md
Merge pull request #18 from erikmontes/patch-1

Minor typo in docs
Add same license from the other clients
Merge pull request #22 from timnguyen/patch-1

Add same license from the other clients