Update build instructions

BUILD.txt

@@ -1,17 +1,18 @@
 //-----------------------------------------------------------------------------
 How to build Crown:
 
-in root directory:
+Type these commands in the root directory (i.e. the same as this file):
 
 1. $ mkdir build
 2. $ cd build
-3. $ cmake ..
-4. $ make terrain
+3. $ cmake .. -DCMAKE_INSTALL_PREFIX=$HOME/your/install/directory
+4. $ make
+5. $ make install
 
 //-----------------------------------------------------------------------------
 In order to select different build configurations you have to do the following:
 
-in root directory:
+In the root directory
 
 1. $ mkdir build
 2. $ cd build