Add instruction to build project in system with

custom Qt installation location.

README.md

@@ -2,15 +2,15 @@
 
 Protobuf plugin to generate Qt classes
 
-# Linux Build
+## Linux Build
 ```bash
 mkdir build
 cd build
-cmake ..
+cmake [-DCMAKE_PREFIX_PATH=<PATH_TO_Qt5Config.cmake_OR_qt5-config.cmake>] ..
 make -j<N>
 ```
 
-# Windows Build
+## Windows Build
 ```bash
 mkdir build
 cd build
@@ -18,7 +18,7 @@ cmake ..
 cmake --build . --target qtprotobuf --config RELEASE
 ```
 
-# Usage
+## Usage
 ```bash
 protoc --plugin=protoc-gen-qtprotobuf=<path/to/bin>/qtprotobuf --qtprotobuf_out=<output_dir> <protofile>.proto
-```
+```