Add README for the compute module.

arrow/src/compute/README.md

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+
+## Apache Arrow Rust Compute Kernels
+
+This module contains analytical kernels that process primarily Arrow
+columnar data; some kernels can process scalar or Arrow-based array
+inputs. These are intended for use inside query engines, data frame libraries,
+etc.
+
+Many kernels have SQL-like semantics in that they perform elementwise or
+scalar operations on whole arrays at a time. Other kernels are not SQL-like
+and compute results that may be a different length or whose results depend on
+the order of the values.
+
+We use the term "kernel" to refer to particular general operation that contains many different functions corresponding to different combinations of types or function behavior options.
+
+Types of functions
+
+* Scalar functions: elementwise functions that perform scalar operations in a
+  vectorized manner. These functions are generally valid for SQL-like
+  context. These are called "scalar" in that the functions executed consider
+  each value in an array independently, and the output array or arrays have the
+  same length as the input arrays. The result for each array cell is generally
+  independent of its position in the array.
+* Vector functions, which produce a result whose output is generally dependent
+  on the entire contents of the input arrays. These functions **are generally
+  not valid** for SQL-like processing because the output size may be different
+  than the input size, and the result may change based on the order of the
+  values in the array. This includes things like array subselection, sorting,
+  hashing, and more.
+* Scalar aggregate functions of which can be used in a SQL-like context