Build instructions enhancement

Current build instruction given in README.md skip the 'cd build' step and also do not provide the cmake --build and cmake --install steps.

Perhaps something like this would be better:

```
git clone https://github.com/quic/quic-teec.git
cd quic-teec
mkdir build
cd build
cmake .. -DCMAKE_TOOLCHAIN_FILE=CMakeToolchain.txt -DBUILD_UNITTEST=ON 
cmake --build . --target install --config Release
```