Readme recommendation about CircleCI appears incorrect

[vdh]

E.g., for CircleCI: before loading/saving your cache run cat patches/* | md5 > patches.hash
and then update your hash key to include a checksum of that file,
{{ checksum "yarn.lock" }}-{{ checksum "patches.hash" }}.

This recommendation is either outdated or incorrect, because md5 appears to be not available in CircleCi:

#!/bin/bash -eo pipefail
cat patches/* | md5 > patches.hash
/bin/bash: md5: command not found

Exited with code exit status 127
CircleCI received exit code 127

[vdh]

It looks like the correct command for CircleCI would be something like md5sum patches/* > patches.hash

[SRandazzo]

md5sum patches/* > patches.hash as suggested above worked for me on CircleCI
Thanks!

[SRandazzo]

@vdh I've taken your recommendation and updated docs in a PR here: #274

thanks again for the help

[SRandazzo]

I've further clarified in my PR when to use md5 vs md5sum

 Create a hash of your patches before loading/saving your cache. If using a Linux machine, run `md5sum patches/* > patches.hash`. If running on a macOS machine,  use `md5 patches/* > patches.hash`