Added mention of release downloads to README.
Added mention of release downloads to README.

JoaoRodrigues/freesasa,mittinatten/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa,mittinatten/freesasa,JoaoRodrigues/freesasa