doc: show prelude contents as book appendix (#390)

Author: molpopgen

book/src/SUMMARY.md

@@ -15,5 +15,6 @@
   - [Working with trees](./tree_sequence_tree.md)
   - [Miscellaneous operations](./tree_sequence_miscellaneous.md)
 
+[Crate prelude](./prelude.md)
 [Changelog](./changelog.md)
 [Migration Guide](./migration_guide.md)

book/src/prelude.md

@@ -0,0 +1,7 @@
+# The `tskit` prelude
+
+The contents of the crate prelude are:
+
+```rust, noplaygound, ignore
+{{#include ../../src/prelude.rs}}
+```