improve README.md

Author: philip82148

README.md

@@ -185,7 +185,7 @@ or
 git submodule add https://github.com/philip82148/cpp-dump
 ```
 
-then
+Then
 
 ```cpp
 #include "path/to/cpp-dump/dump.hpp"
@@ -451,6 +451,18 @@ int main() {
 }
 ```
 
+Then
+
+```shell
+g++ ./main.cpp -D DEFINED_ONLY_IN_LOCAL
+```
+
+or
+
+```shell
+clang++ ./main.cpp -D DEFINED_ONLY_IN_LOCAL
+```
+
 ## Supported types
 
 Both dump functions dump variables recursively, so they can dump nested variables of any combination of types in the table below.