Add 9legacy requirements to README

[fhs]

9legacy needs these changes from 9front:

• rc-line-split: implement delim{...} syntax for rc
• walk: port walk command to 9legacy.
• hold command

I'll update the list as I find more issues.

[oridb]

FYI, I tend to be much better about checking email than github issues.

Anyways, done :)