Add avoiding Lazy Continuation Lines to the Contributor Guide

[wwarriner]

On the subject of "Lazy Continuation Lines". They are not strictly against the specification of Markdown, and so are not (yet) caught by Markdown Lint nor by the automatic formatter. There is an issue to add an optional rule for this to Markdown Lint: DavidAnson/markdownlint#977

Here is what it looks like:

this is a lazy continuation line and will be part of the previous line
  so will this
                   so will this

- this is an unordered list item
lazy continuation line
  and this
                 and this

lists aren't
  required
for this```

Each of the examples above have all of the text rendered into continuous paragraphs, like below.

```1. this is an ordered list item this is a lazy continuation line and will be part of the previous line so will this so will this

- this is an unordered list item lazy continuation line and this and this

lists aren't required for this```

Do not use lazy continuation lines as they create hard-to-maintain inconsistencies in the documentation and appear ambiguous in terms of writer intent. Based on your intent, either explicitly connect the lazy continuation to the previous item by removing the newline, or explicitly include a blank line between the list item and the continuation line.

Using the ordered list example, one or the other of the two options below.

```1. this is an ordered list item this is a lazy continuation line and will be part of the previous line so will this so will this

1. this is an ordered list item

    this is a lazy continuation line and will be part of the previous line
 
    so will this
    
    so will this```