doc: add migration guide (#376)

Author: molpopgen

MIGRATION_GUIDE.md

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+# Migration guide
+
+This document helps with migration to newer versions of `tskit-rust`.
+This document will not be exhaustive.
+Rather, it will focus on highlights or "must know" items.
+
+Regarding breaking changes:
+
+* This document will discuss breaking changes requiring change in client code.
+* Other changes listed in the change log as breaking are considered "technically breaking
+  but unlikely to affect anyone in practice". Please file an issue if we are wrong!
+
+Acronyms used:
+
+* UB = undefined behavior.
+
+## v0.11.0
+
+### Bug fixes
+
+* Issue [363](https://github.com/tskit-dev/tskit-rust/issues/363), which was introduced in PR [300](https://github.com/tskit-dev/tskit-rust/pull/300).
+  The bug resulted in UB when advancing trees for release builds.
+
+### Breaking changes
+
+#### New error variant.
+
+The error enum now includes `LibraryError`.
+Any code previously matching on errors will now break because the match is no longer exhaustive.
+
+#### Row getters
+
+This change occurred over several PR.
+
+All "getter" functions for table rows now return [`Option`](https://doc.rust-lang.org/std/option/enum.Option.html) instead of [`Result`](https://doc.rust-lang.org/std/result/enum.Result.html) when a row index is out of bounds.
+These functions now return `None` for the out of bounds case.
+This behavior change brings the API in line with other rust APIs.
+
+To update:
+
+* If `None` should represent an error for your use case, use [`Option::ok_or_else`](https://doc.rust-lang.org/std/option/enum.Option.html#method.ok_or_else).
+  In the case of a previous `Result<Option<_>, _>` that is now an `Option<Result<_, _>>`, use [`transpose`](https://doc.rust-lang.org/std/option/enum.Option.html#method.transpose).
+
+#### Lifetime annotations of table types
+
+PR [#373](https://github.com/tskit-dev/tskit-rust/pull/373).
+
+Previously, a non-owning view of an edge table had a type `EdgeTable<'a>`.
+Those lifetimes are no longer necessary, which is a big design win.
+
+Two traits, `TableAccess` and `NodeListRowGenerator`, have been removed.
+The `TableCollection` and `TreeSequence` types now contain that functionality as
+[`Deref`](https://doc.rust-lang.org/std/ops/trait.Deref.html) targets.
+
+Removing these traits implies:
+
+* Their removal from the crate prelude.
+* They can no longer be used as trait bounds.
+
+Some views of a node table that required mutability broke due to these changes.
+A new function in the `Deref` target is `nodes_mut(&mut self)` addresses this breakage.