docs: add why section to README

README.md

@@ -5,10 +5,33 @@
 [![crates.io](https://img.shields.io/crates/v/narrow.svg)](https://crates.io/crates/narrow)
 [![docs.rs](https://docs.rs/narrow/badge.svg)](https://docs.rs/narrow)
 
-An experimental (work-in-progress) implementation of [Apache Arrow](https://arrow.apache.org).
+An experimental (work-in-progress) statically typed implementation of [Apache Arrow](https://arrow.apache.org).
 
 This crate provides types to support reading and writing instances of abstract data types in Arrow's in-memory data structures.
 
+## Why
+
+The development of this crate started because:
+
+- The [arrow](https://docs.rs/arrow) crate provides APIs that mostly make sense when array types are only known at runtime, however, for applications with [types known at compile-time](https://docs.rs/arrow/latest/arrow/#type-erasure--trait-objects), using [trait objects](https://doc.rust-lang.org/book/ch17-02-trait-objects.html) and [downcasting](https://docs.rs/arrow/latest/arrow/array/fn.downcast_array.html) is not ergonomic.
+- Builders for [nested](https://docs.rs/arrow/latest/arrow/datatypes/enum.DataType.html#method.is_nested) array types are [complex](https://docs.rs/arrow/latest/arrow/array/struct.StructBuilder.html).
+
+### Goals and non-goals
+
+#### Goals
+
+- Provide safe, efficient, statically typed Arrow array implementations
+- Enable everyone using Rust to easily benefit from the Arrow ecosystem
+- Provide zero-copy interop with the [arrow](https://docs.rs/arrow) crate
+- Support custom buffer implementations e.g. to support accelerators
+- Explore expressing Arrow concepts using the Rust type system, and mapping Rust concepts to Arrow
+
+#### Non-goals
+
+- Support arbitrary array types at runtime (the [arrow](https://docs.rs/arrow) crate supports this use case)
+- Provide compute kernels
+- Replace other Arrow implementations
+
 ## Docs
 
 - [Docs (release)](https://docs.rs/narrow)