doc(README): distinguish LLVM 19 vs 20 in fpm test

This commit updates the README.md file to explain that two
experimental flags required with LLVM 19 are not required with
LLVM 20.

Author: rouson

README.md

@@ -103,9 +103,14 @@ fpm test --compiler ifx --profile release --flag "-coarray -DASSERT_MULTI_IMAGE"
 ```
 
 ### Building and testing with the LLVM `flang-new` compiler
+#### LLVM 19 Version
 ```
 fpm test --compiler flang-new --flag "-mmlir -allow-assumed-rank -O3"
 ```
+#### LLVM 20 or later
+```
+fpm test --compiler flang-new --flag "-O3"
+```
 
 ### Building and testing with the Numerical Algorithms Group (NAG) compiler
 ```