INSTALL.md: Add further explanations

Add the actual commands to use to compile GAP as they are explained in README.md

INSTALL.md

@@ -609,13 +609,31 @@ a terminal in order to make all required tools available via the command line:
 
      xcode-select --install
 
-You might want to consider using Readline by installing it via:
+You might want to consider using GNU readline by installing it via:
 
  * using Homebrew: `brew install readline`
  * using Fink: `fink install readline7`
  * using MacPorts: `port install readline`
+
+After that you have to compile GAP and tell it where to find your installation of GNU readline
+using the following commands.
 
-After that follow the instruction in section 5 about how to use Readline.
+For Homebrew, use these commands:
+
+    ./configure --with-readline=$(brew --prefix)/opt/readline
+    make
+
+For Fink, use these commands:
+
+    ./configure CPPFLAGS=-I/sw/include LDFLAGS=-L/sw/lib
+    make
+
+For MacPorts, use these commands:
+
+    ./configure CPPFLAGS=-I/opt/local/include LDFLAGS=-L/opt/local/lib
+    make
+
+For further information on how to use GNU readline, refer to section 5 above.
 
 Now simply follow the Unix installation instructions to compile and start
 GAP and then it will run in this Terminal window.