Add exponential likelihood to the documentation

docs/likelihood.txt

@@ -226,6 +226,10 @@ As well as the Gaussian likelihood defined above, tupak provides
 the following common likelihood functions:
 
 .. autoclass:: tupak.core.likelihood.PoissonLikelihood
+   :members:
+
+.. autoclass:: tupak.core.likelihood.ExponentialLikelihood
+   :members:
 
 Likelihood for transient gravitational waves
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