Update doc about graphviz errors

README.md

@@ -31,6 +31,16 @@ You can install these dependencies with the following command:
     sudo apt-get install python-gtksourceview2 python2.7-dev libxml2-dev libxslt1-dev \
     libxslt1-dev libgraphviz-dev pkg-config python-glade2 python-gtk2 python-tk
 
+On some old systems you may have to install also `python-pip`.
+When installing pygraphviz on some systems, you may have this error:
+
+    _graphviz.so: undefined symbol: Agundirected
+
+...which is solved by:
+
+    pip install --upgrade pygraphviz --install-option="--include-path=/usr/include/graphviz" \
+    --install-option="--library-path=/usr/lib/graphviz/"
+
 ### Red Hat-like systems (Fedora/CentOS)
 
 * python-devel

library/README.md

@@ -31,6 +31,16 @@ You can install these dependencies with the following command:
     sudo apt-get install python-gtksourceview2 python2.7-dev libxml2-dev libxslt1-dev \
     libxslt1-dev libgraphviz-dev pkg-config python-glade2 python-gtk2 python-tk
 
+On some old systems you may have to install also `python-pip`.
+When installing pygraphviz on some systems, you may have this error:
+
+    _graphviz.so: undefined symbol: Agundirected
+
+...which is solved by:
+
+    pip install --upgrade pygraphviz --install-option="--include-path=/usr/include/graphviz" \
+    --install-option="--library-path=/usr/lib/graphviz/"
+
 ### Red Hat-like systems (Fedora/CentOS)
 
 * python-devel