Document building QtC with system qbs

Change-Id: I551d2a909db7875183b74160cedc958b4a1d5e8c
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

README.md

@@ -26,6 +26,7 @@ Prerequisites:
 * On Mac OS X: latest Xcode
 * On Linux: g++ 4.8 or later
 * LLVM 3.9.0 or later (optional, needed for the Clang Code Model)
+* Qbs 1.7.x (optional, sources also contain Qbs itself)
 
 The installed toolchains have to match the one Qt was compiled with.
 
@@ -33,6 +34,8 @@ You can build Qt Creator with
 
     # Optional, needed for the Clang Code Model:
     export LLVM_INSTALL_DIR=/path/to/llvm (or "set" on Windows)
+    # Optional, needed to let the QbsProjectManager plugin use system Qbs:
+    export QBS_INSTALL_DIR=/path/to/qbs
 
     cd $SOURCE_DIRECTORY
     qmake -r