Document the feedback functionality

[oleks]

The parsec parser does allow to provide a "Feedback judgement" like this:

# Assignment 1: 10/10
## Feedback:
  * Good work!
## Task 1: 10/10

Then, we can extract the feedback using the CLI:

$ remarks feedback remarks.mrk
# Assignment 1

  * Good work!

However, the README makes no mention of Feeedback judgements, while still mentioning the feedback CLI command, which yields nothing if no Feedback judgements are present.

My immediate questions are:

1. Is it supposed to be allowed that you have multiple Feedback judgements in a remarks document? (That would make sense under the original design.)
2. Is a Feedback judgement supposed to be placed at the top of a judgement, above some other other sub-judgements?
3. Is it intended that there can be multiple Feedback judgements under the same judgement?

Wrt. question 1, remarks currently does accept the following:

# Assignment 1: 0/10
## Feedback:
  * Good work!
## Task 1: 10/10
# Assignment 2: 0/10
## Feedback:
  * Bad work :-(
## Task 1: 0/10

However, it yields:

$ remarks feedback remarks.mrk
# Assignment 1

  * Good work!# Assignment 2

                * Bad work :-(

Which is not ideal ;-)

Wrt. question 2, the following yields a syntax error:

# Assignment 1: 0/10
## Feedback:
  * Good work!

In particular,

$ remarks feedback remarks.mrk
"remarks.mrk" (line 4, column 1):
unexpected end of input
expecting white space or "##"