Add info for -j4 explain

README.md

@@ -14,7 +14,7 @@ sudo apt-get install build-essential pkg-config libc6-dev m4 g++-multilib \
       bsdmainutils automake curl unzip nano libsodium-dev
 git clone https://git.hush.is/hush/hush3.git
 cd hush3
-./build.sh -j4
+./build.sh -j4  #4 for use 4 cores
 ```
 
 More details can be found [here](https://git.hush.is/hush/hush3/src/branch/master/INSTALL.md)