Add a doc describing how to add custom suggestion strategies. This was very hard to figure out without guidance.

How-to-Add-Custom-Suggestion-Strategy.md

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+## Adding a Custom Suggestion Strategy
+
+1. Add a `_zsh_autosuggest_strategy_<your custom strategy name>` function to your `.zshrc`. It must set the global variable `suggestion`
+2. Export the `ZSH_AUTOSUGGEST_STRATEGY`
+
+#### Example
+This example applies the history strategy first and if it fails to find a match then it applies the custom strategy
+
+```
+_zsh_autosuggest_strategy_custom() {
+  typeset -g suggestion
+  suggestion=`./my-custom-suggestion-script.sh "$1"`
+}
+
+export ZSH_AUTOSUGGEST_STRATEGY=(history custom)
+```