Edit README

README.md

@@ -7,7 +7,6 @@ __Neither QA, cuts nor macro nor any logic are expected in current package.__
 ```
 cd < DataTree directory >/
 ```
-To build with ROOT v6 uncomment line 26: "include(${ROOT_USE_FILE})" in CMakeLists.txt
 
 Make build directory:
 
@@ -16,7 +15,33 @@ mkdir build
 cd build
 ```
 
-Link all source files via cmake:
+Initialize your ROOT environment with `source <path-to-root>/bin/thisroot.sh`. Make sure, your compiler is the same as it was for root. This is important if you would like compile DataTree for i686 (32 bit), for instance. Otherwise you will get
+
+```
+/usr/bin/ld: skipping incompatible /usr/lib64/libdl.so when searching for -ldl
+/usr/bin/ld: skipping incompatible /usr/lib64/libdl.a when searching for -ldl
+/usr/bin/ld: skipping incompatible /usr/lib64/librt.so when searching for -lrt
+/usr/bin/ld: skipping incompatible /usr/lib64/librt.a when searching for -lrt
+```
+
+On lxplus one can compiler used for ROOT with
+
+```
+$ which root
+/afs/cern.ch/sw/lcg/app/releases/ROOT/5.34.19/i686-slc6-gcc47-opt/root/bin/root
+```
+
+*i686-slc6-gcc47-opt* is the needed one. It won't work for the case of default root, but in this case it is assumed it was built with default environment so the next step is not neccessary.
+
+To use it, type
+
+```
+source source /afs/cern.ch/sw/lcg/contrib/gcc/4.7/i686-slc6-gcc47-opt/setup.sh
+export CC=/afs/cern.ch/sw/lcg/contrib/gcc/4.7/i686-slc6-gcc47-opt/bin/gcc
+export CXX=/afs/cern.ch/sw/lcg/contrib/gcc/4.7/i686-slc6-gcc47-opt/bin/g++
+``` 
+
+Prepare build
 
 ```
 cmake [-DCMAKE_INSTALL_PREFIX=\<desired location\>] ../src/