missing utils/README.md

The installed actions for utils package are not documented

```
$ wsk package get /whisk.system/utils --summary
package /whisk.system/utils: Building blocks that format and assemble data
action /whisk.system/utils/date: Current date and time
action /whisk.system/utils/head: Extract prefix of an array
(parameters: lines, num)
action /whisk.system/utils/split: Split a string into an array
(parameters: payload, separator)
action /whisk.system/utils/sort: Sorts an array
(parameters: lines)
action /whisk.system/utils/echo: Returns the input
(parameters: payload)
action /whisk.system/utils/cat: Concatenates input into a string
(parameters: lines)
```

Start with the utils package and compare its installed actions with the output of `wsk package get /whisk.system/utils --summary`. Add the missing utils/README.md documenting date, head, split, sort, echo, and cat, including the parameters shown; done means each listed action is covered.

Written by the indexing model from the issue text.