github
Document --alter
It seems to me that nowhere in the documentation do you explain how to provide the actual migration/ALTER TABLE statement to gh-ost. A few places use --alter="engine=innodb" in examples, but it is far from obvious that "engine=innodb" is supposed to be the migration (as in ALTER TABLE my_table engine=innodb), rather than some configuration option.
Not even the command-line-flags.md mention --alter. Nor does the announcement blog post. Seem you need to look at third party blog posts, or be familiar with for example pt-online-schema-change, to know that --alter is used to pass the migration itself.
Thank you! Would you be open to submit a PR with changes to the docs, with a few examples?
