Add a blurb about min Golang version in README

[pRizz]

The default golang version on AWS images utilizing yum install go point to 1.9 which causes errors during runtime. Adding this blurb should help people resolve this issue.

Here is a sample of the errors if running 1.9

go run openbazaard.go start
# github.com/OpenBazaar/openbazaar-go/vendor/github.com/OpenBazaar/multiwallet/service
vendor/github.com/OpenBazaar/multiwallet/service/wallet_service.go:153:28: undefined: math.Round
vendor/github.com/OpenBazaar/multiwallet/service/wallet_service.go:428:14: undefined: math.Round
vendor/github.com/OpenBazaar/multiwallet/service/wallet_service.go:464:14: undefined: math.Round
# github.com/OpenBazaar/openbazaar-go/vendor/gx/ipfs/QmT4U94DnD8FRfqr21obWY32HLM5VExccPKMjQHofeYqr9/go-multiaddr
vendor/gx/ipfs/QmT4U94DnD8FRfqr21obWY32HLM5VExccPKMjQHofeYqr9/go-multiaddr/codec.go:141:10: undefined: strings.Builder
vendor/gx/ipfs/QmT4U94DnD8FRfqr21obWY32HLM5VExccPKMjQHofeYqr9/go-multiaddr/component.go:69:8: undefined: strings.Builder
vendor/gx/ipfs/QmT4U94DnD8FRfqr21obWY32HLM5VExccPKMjQHofeYqr9/go-multiaddr/component.go:76:32: undefined: strings.Builder

[placer14]

Thanks for offering the document improvements.