docs/alloy-models: add initial README.md for Alloy models

Author: Roasbeef

docs/alloy-models/README.md

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+# Alloy Models
+
+This folder houses "lightweight formal models" written using the
+[Alloy](https://alloytools.org/about.html) model checker and language.
+
+Compared to typical formal methods, Alloy is a _bounded_ model checker,
+considered a [lightweight
+method](https://en.wikipedia.org/wiki/Formal_methods#:~:text=As%20an%20alternative,Tools.%5B31)
+to formal analysis. Lightweight formal methods are easier to use than fully
+fledged formal methods as rather than attempting to prove the that a model is
+_always_ correct (for all instances), they instead operate on an input of a set
+of bounded parameters and iterations. These models can then be used to specify
+a model, then use the model checker to find counter examples of a given
+assertion. If a counter example can't be found, then the model _may_ be valid.
+
+Alloy in particular is an expressive, human readable language for formal
+modeling. It also has a nice visualizer which can show counter examples, aiding
+in development and understanding.
+
+Alloy is useful as when used during upfront software design (or even after the
+fact), one can create a formal model of a software system to gain better
+confidence re the _correctness_ of a system. The model can then be translated
+to code. Many times, writing the model (either before or after the code) can
+help one to better understand a given software system. Models can also be used
+to specify protocols in p2p systems such as Lightning (as it [supports temporal
+logic](https://alloytools.org/alloy6.html#:~:text=Meaning%20of%20temporal%20connectives),
+which enables creation of state machines and other interactive transcripts),
+serving as a complement to a normal prosed based specification.
+
+## How To Learn Alloy?
+
+We recommend the following places to learn Alloy:
+  * [The online tutorial for Alloy 4.0](https://alloytools.org/about.html):
+     * Even though this is written with Alloy 4.0 (which doesn't support
+       temporal logic), the tutorial is very useful, as it introduces
+       fundamental concept of Alloy, using an accessible example based on a file
+       system.
+
+  * [Alloy Docs](https://alloy.readthedocs.io/en/latest/index.html):
+     * This serves as a one stop shop for reference to the Alloy language. It
+       explains all the major syntax, tooling, and also how to model time in
+       Alloy.
+
+  * [Formal Software Design with Alloy 6](https://haslab.github.io/formal-software-design/index.html): 
+     * A more in depth tutorial which uses Alloy 6. This tutorial covers more
+       advanced topics such as [event
+       reification](https://alloytools.discourse.group/t/modelling-a-state-machine-in-electrum-towards-alloy-6/88).
+       This tutorial is also very useful, as it includes examples for how to
+       model interactions in a p2p network, such as one peer sending a message
+       to another.