Add instructions about initializing variable

[issmirnov]

Many users use modular configuration loaders for zshrc. Adding a note to users explaining how to define ZSH_HIGHLIGHT_PATTERNS before sourcing zsh-syntax-highlighting. The typeset is important, since simply setting the variable doesn't have the desired effect.

[issmirnov]

See discussion in #524

[danielshahaf]

See discussion in #524

The cases are similar but there's an important distinction: #524 is about an array (typeset -ga) and this issue is about an associative array (typeset -gA). Just writing foo=(1 2 3) implicitly declares foo as a normal array; typeset -A must be used before assigning to an associative array.

(I don't use the pattern highlighter so I hoped one of the other devs would review this patch.)