Small fix to doc

Former-commit-id: 78f7bed

Author: neubig

doc/source/python.rst

@@ -59,9 +59,9 @@ The following is a list of all the commands needed to perform a manual install:
     cd dynet
     mkdir build
     cd build
-    # without GPU support:
+    # without GPU support (if you get an error that Eigen cannot be found, try using the full path to Eigen)
     cmake .. -DEIGEN3_INCLUDE_DIR=../../eigen -DPYTHON=`which python`
-    # or with GPU support:
+    # or with GPU support (if you get an error that Eigen cannot be found, try using the full path to Eigen)
     cmake .. -DEIGEN3_INCLUDE_DIR=../../eigen -DPYTHON=`which python` -DBACKEND=cuda
 
     make -j 2 # replace 2 with the number of available cores