Add reference to RTD manual

README.md

@@ -28,9 +28,12 @@ functions or small PAO basis sets are also possible.
 CONQUEST reads pseudopotentials produced by Don Hamann's
 [ONCVPSP](http://www.mat-simresearch.com) code, and is fully
 compatible with the the [PseudoDojo](http://www.pseudo-dojo.org) 
-and [SG15](http://www.quantum-simulation.org/potentials/sg15_oncv/) databases.  It can also read the .ion
+and [SG15](http://www.quantum-simulation.org/potentials/sg15_oncv/)
+databases. It can also read the .ion
 files produced by Siesta, using both the pseudopotentials and
-pseudo-atomic orbitals (PAOs) in those files.
-
-CONQUEST integrates with
+pseudo-atomic orbitals (PAOs) in those files.  CONQUEST integrates with
 [ASE](https://wiki.fysik.dtu.dk/ase/index.html), the atomic simulation environment.
+
+The manual is provided with the release (see the ``docs`` directory)
+but is also available on
+[ReadTheDocs](https://conquest.readthedocs.io/en/latest/).