added workaround libgcc error

docs/setup.rst

@@ -18,6 +18,15 @@ the following python packages must be installed::
     pip install future
     pip install python-igraph
 
+**Hint:** If later, during usage of the igraph library, an error like this::
+
+    ImportError: /home/gks/anaconda3/lib/python3.6/site-packages/igraph/_igraph.cpython-36m-x86_64-linux-gnu.so: undefined symbol: _ZNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEC1Ev
+
+occurs, then the libgcc package has to be installed/upgraded::
+
+    conda install libgcc
+
+
 * MongoDB
 
 If your system is Ubuntu and you have administrator permissions you can install