expanding the readme

Author: lemire

README.md

@@ -2,11 +2,21 @@
 ## Fast WHATWG forgiving-base64 in C#
 
 The C# standard library has fast (SIMD-based) base64 encoding functions, but it lacks
-base64 decoding function. The initial work that lead to the fast functions in the runtime
-was carried out by [gfoidl](https://github.com/gfoidl/Base64).
+really fast base64 decoding function. The initial work that lead to the fast functions in the runtime
+was carried out by [gfoidl](https://github.com/gfoidl/Base64). 
+
+-  There are accelerated base64 functions for UTF-8 inputs in the .NET runtime, but they are not optimal: 
+we can make them 50% to 2x or 3x faster.
+- There is no accelerated base64 functions for UTF-16 inputs (e.g., `string` types). We can be 2x faster
+or more.
 
 The goal of this project is to provide the fast WHATWG forgiving-base64 algorithm already
-used in major JavaScript runtimes (Node.js and Bun) to C#. It would complete the existing work.
+used in major JavaScript runtimes (Node.js and Bun) to C#.
+
+Importantly, we only focus on base64 decoding. It is a more challenging problem than base64 encoding because
+of the presence of allowable white space characters and the need to validate the input. Indeed, all
+inputs are valid for encoding, but only some inputs are valid for decoding. Having to skip white space 
+characters makes accelerated decoding somewhat difficult.
 
 
 ## Requirements