Documentation updates in README.

README.md

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 # Various Python tools for OpenMM
 
-## `openmmtools.testsystems`
+## Integrators
 
-This repository contains a suite of molecular systems that can be used
-for the testing of various molecular mechanics related software.  The
-idea is that this repository will host a number classes that generate
-OpenMM objects for simulating the desired systems.
+This repository contains a number of additional integrators for OpenMM in `openmmtools.integrators`, including
+* `MTSIntegrator` - a multiple timestep integrator
+* `DummyIntegrator` - a "dummy" integrator that does not update positions
+* `GradientDescentMinimizationIntegrator` - a simple gradient descent minimizer (without line search)
+* `VelocityVerletIntegrator` - a velocity Verlet integrator
+* `AndersenVelocityVerletIntegrator` - a velocity Verlet integrator with Andersen thermostat using per-particle collisions
+* `MetropolisMonteCarloIntegrator` - a Metropolis Monte Carlo integrator that uses Gaussian displacement trials
+* `HMCIntegrator` - a hybrid Monte Carlo (HMC) integrator
+* `GHMCIntegrator` - a generalized hybrid Monte Carlo (GHMC) integrator
+* `VVVRIntegrator` - a velocity Verlet with velocity randomization (VVVR) integrator
 
-These classes will also contain the member functions that calculate known
-analytical properties of these systems, enabling the proper testing.
+## Test system suite
 
-## `scripts`
+The `openmmtools.testsystems` module contains a large suite of test systems---including many with simple exactly-computable properties---that can be used to test molecular simulation algorith,s
 
-A script that may be useful in testing your OpenMM installation is installed:
+## OpenMM testing scripts
+
+`scripts/` contains a script that may be useful in testing your OpenMM installation is installed:
 
 * `test-openmm-platforms` will test the various platforms available to OpenMM to ensure that all systems in `openmmtools.testsystems` give consistent potential energies.
 If differences in energies in excess of `ENERGY_TOLERANCE` (default: 0.06 kcal/mol) are detected, these systems will be serialized to XML for further debugging.
+
+This is installed onto the command line when the repository is installed.