Readme recommendation about CircleCI appears incorrect

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This recommendation is either outdated or incorrect, because `md5` appears to be not available in CircleCi: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