drop default key bindings (move them into README)

https://github.com/sorin-ionescu/prezto/issues/422

https://github.com/sorin-ionescu/prezto/commit/3c5363ef9bcfda4cc5cd46ca85a7109f783c9539
Merge branch 'master' of https://github.com/zsh-users/zsh-history-substring-search

* 'master' of https://github.com/zsh-users/zsh-history-substring-search:
  drop default key bindings (move them into README)

Conflicts:
	zsh-history-substring-search.zsh