Fix link to virtualenv installation instructions.
Fix link to virtualenv installation instructions.

Also made link to detailed doc instructions more prominent
jas14/khmer,ged-lab/khmer,ged-lab/khmer,souravsingh/khmer,F1000Research/khmer,Winterflower/khmer,souravsingh/khmer,jas14/khmer,kdmurray91/khmer,souravsingh/khmer,F1000Research/khmer,Winterflower/khmer,kdmurray91/khmer,jas14/khmer,F1000Research/khmer,Winterflower/khmer,ged-lab/khmer,kdmurray91/khmer