Document the vendor folder

vendor/README.md

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+The main purpose of this folder is to drop here Emacs packages
+that for some reason are not available via `package.el` repositories
+(e.g. you might want some package available only on EmacsWiki).
+
+Any code that you place here will be available via `require`.
+Within Prelude's code you can refer to this folder via
+`prelude-vendor-dir`.