Added C++20 mathematical constants.

CPP20.md

@@ -22,6 +22,7 @@ C++20 includes the following new library features:
 - [synchronized buffered outputstream](#synchronized-buffered-outputstream)
 - [std::span](#stdspan)
 - [bit operations](#bit-operations)
+- [math constants](#math-constants)
 
 ## C++20 Language Features
 
@@ -418,6 +419,13 @@ std::popcount(1u); // 1
 std::popcount(0b1111'0000u); // 4
 ```
 
+### math constants
+Mathematical constants including PI, Euler's number, etc. defined in the `<numbers>` header.
+```c++
+std::numbers::pi; // 3.14159...
+std::numbers::e; // 2.71828...
+```
+
 ## Acknowledgements
 * [cppreference](http://en.cppreference.com/w/cpp) - especially useful for finding examples and documentation of new library features.
 * [C++ Rvalue References Explained](http://thbecker.net/articles/rvalue_references/section_01.html) - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.