introduce the basic structure of a book (very much a WIP)

book/.gitignore

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+book

book/book.toml

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+[book]
+authors = ["Niko Matsakis"]
+multilingual = false
+src = "src"
+title = "Salsa"

book/src/SUMMARY.md

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+# Summary
+
+- [About salsa](./about_salsa.md)
+- [How to use Salsa](./how_to_use.md)
+- [How Salsa works](./how_salsa_works.md)
+- [Common patterns](./common_patterns.md)
+- [YouTube videos](./videos.md)

book/src/about_salsa.md

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+# About salsa
+
+Salsa is a Rust framework for writing incremental, on-demand programs
+-- these are programs that want to adapt to changes in their inputs,
+continuously producing a new output that is up-to-date. Salsa is based
+on the the incremental recompilation techniques that we built for
+rustc, and many (but not all) of its users are building compilers or
+other similar tooling.
+
+If you'd like to learn more about Salsa, you can check out [the [Hello
+World
+example](https://github.com/salsa-rs/salsa/blob/master/examples/hello_world/main.rs)
+in the repository, or watch some of our [YouTube videos](./videos.md).
+
+If you'd like to chat about Salsa, or you think you might like to
+contribute, please jump on to our Zulip instance at
+[salsa.zulipchat.com](https://salsa.zulipchat.com/).

book/src/common_patterns.md

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+# Common patterns

book/src/how_salsa_works.md

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+# How Salsa works

book/src/how_to_use.md

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+# How to use Salsa

book/src/videos.md

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+# YouTube videos
+
+There are currently two videos about Salsa available:
+
+- [How Salsa Works](https://youtu.be/_muY4HjSqVw), which gives a
+  high-level introduction to the key concepts involved and shows how
+  to use salsa;
+- [Salsa In More Depth](https://www.youtube.com/watch?v=i_IhACacPRY),
+  which digs into the incremental algorithm and explains -- at a
+  high-level -- how Salsa is implemented.
+