There are now three plugins which can turn the preview+ button
below every page edit box into a spellcheck function.

Wether you have a working 'aspell' or 'ispell' on your system, or
the PHP internal aspell functions, you must load the according
prev << "edit/lock"
next >> "edit/spam_deface"

You cannot modify the README.plugins file, but anyhow any ideas or suggestion should as usually get filed on BugReports, UserSuggestions or even better the README.plugins.Discussion.