Document --exclude-archs flag

[filipenevola]

We should document this flag in the run command.

From History.md

You can now pass an --exclude-archs option to the meteor run and meteor test commands to temporarily disable building certain web architectures. For example, meteor run --exclude-archs web.browser.legacy. Multiple architectures should be separated by commas. This option can be used to improve (re)build times if you're not actively testing the excluded architectures during development. Feature meteor/meteor-feature-requests#333 PR meteor/meteor#10824