add module documentation

mythril/laser/ethereum/plugins/__init__.py

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+""" Laser plugins
+
+This module contains everything to do with laser plugins
+
+Laser plugins are a way of extending laser's functionality without complicating the core business logic.
+Different features that have been implemented in the form of plugins are:
+- benchmarking
+- path pruning
+
+Plugins also provide a way to implement optimisations outside of the mythril code base and to inject them.
+The api that laser currently provides is still unstable and will probably change to suit our needs
+as more plugins get developed.
+
+For the implementation of plugins the following modules are of interest:
+- laser.plugins.plugin
+- laser.plugins.signals
+- laser.svm
+
+Which show the basic interfaces with which plugins are able to interact
+"""