Updated the readme to note current gcc and Clang limitations.

README.md

@@ -1,6 +1,7 @@
 # char8_t backward compatibility remediation
 
 - [Introduction](#introduction)
+- [Requirements](#requirements)
 - [Remediation examples](#remediation-examples)
   - [String literals](#string-literals)
   - [Array initialization](#array-initialization)
@@ -53,6 +54,20 @@ using u8string = std::string;
 ```
 
 
+# Requirements
+The implementation of
+[P0732R2](https://wg21.link/p0732 "Class Types in Non-Type Template Parameters")
+in gcc 9.1.0 is incomplete and incorrectly rejects literal operator templates
+declared with non-type template parameters of literal class type.  This issue is
+tracked by
+[bug 88095](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88095 "class nontype template parameter UDL string literals doesn't accepts deduction placeholder").  This issue currently prevents use of the `U8`
+macro when compiling for C++20.  A patch is available.
+
+[P0732R2](https://wg21.link/p0732 "Class Types in Non-Type Template Parameters")
+has not yet been implemented as of Clang 8.  As a result, the `U8` macro can not
+currently be used in `constexpr` context with Clang when compiling for C++20.
+
+
 # Remediation examples