repo: make ripgrep build with the new organization

crates/core/README.md

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+ripgrep core
+------------
+This is the core ripgrep crate. In particular, `main.rs` is where the `main`
+function lives.
+
+Most of ripgrep core consists of two things:
+
+* The definition of the CLI interface, including docs for every flag.
+* Glue code that brings the `grep-matcher`, `grep-regex`, `grep-searcher` and
+  `grep-printer` crates together to actually execute the search.
+
+Currently, there are no plans to make ripgrep core available as an independent
+library. However, much of the heavy lifting of ripgrep is done via its
+constituent crates, which can be reused independent of ripgrep. Unfortunately,
+there is no guide or tutorial to teach folks how to do this yet.