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`](https://github.com/github/gh-ost/blob/master/doc/command-line-flags.md) mention `--alter`. Nor does the [announcement blog post](https://github.blog/2016-08-01-gh-ost-github-s-online-migration-tool-for-mysql/). Seem you need to look at third party blog posts, or be familiar with for example [pt-online-schema-change](https://www.percona.com/doc/percona-toolkit/2.1/pt-online-schema-change.html), to know that `--alter` is used to pass the migration itself.