Move book dev instructions out of top-level readme

book/README.md

@@ -0,0 +1,26 @@
+Building the Solana book
+---
+
+Install mdbook:
+
+```bash
+cargo install mdbook
+```
+
+Run any Rust tests in the markdown:
+
+```bash
+make test
+```
+
+Render markdown as HTML:
+
+```bash
+make build
+```
+
+Render and view the book:
+
+```bash
+make open
+```