fixes: picojson.h: No such file or directory (#3067)

Author: pavelskipenes

readme.md

@@ -118,7 +118,7 @@ Flags:
 If you just want to build Cppcheck without dependencies then you can use this command:
 
 ```shell
-g++ -o cppcheck -std=c++11 -Iexternals -Iexternals/simplecpp -Iexternals/tinyxml2 -Ilib cli/*.cpp lib/*.cpp externals/simplecpp/simplecpp.cpp externals/tinyxml2/*.cpp
+g++ -o cppcheck -std=c++11 -Iexternals -Iexternals/simplecpp -Iexternals/tinyxml2 -Iexternals/picojson -Ilib cli/*.cpp lib/*.cpp externals/simplecpp/simplecpp.cpp externals/tinyxml2/*.cpp
 ```
 
 If you want to use `--rule` and `--rule-file` then dependencies are needed: