fix: add middleware directory missed by #792 to project readme

[scscgit]

• PR feat: update design #792 has removed the individual README.md files and moved their contents (after slight changes) to the README.md in the project root. However, the middleware section is missing, and there is no hint about this being intentional, causing project inconsistency if we follow these changes in our projects. At the very least, there is a redundant newline in (alphabetical) place where this is missing, which could be removed instead.
• Please confirm if we want to re-phrase this section, I just changed .\nMiddleware let you to , which lets you to remove a redundance & to align the let with Nuxt's official . Middleware lets you. Other than the linked directory structure documentation, which was updated from https://nuxtjs.org/guide/routing#middleware to https://nuxtjs.org/docs/2.x/directory-structure/middleware, this middleware is also documented by https://nuxtjs.org/examples/middleware-router and https://nuxtjs.org/examples/middleware-named