Add description of input to README.md

Compilation requires the Glasgow Haskell Compiler [ghc >= 7.8.1](https://www.haskell.org/ghc/)  and the build system [cabal >= 1.22](https://www.haskell.org/cabal/).

Input

------

Peregrine takes a single population protocol as input.

The following example shows how a protocol is encoded:

```

population protocol "Majority Protocol" {

    states { a b a_small b_small}

    transitions { t1 t2 t3 t4 }

    arcs { { a, b } -> t1 -> { a_small, b_small }

           { b, a_small } -> t2 -> { b, b_small }

           { a, b_small } -> t3 -> { a, a_small }

           { a_small, b_small } -> t4 -> { a_small, a_small }

         }

    initial { a b }

    true { a a_small }

    false { b b_small } 

}

```

* After the key word *population protocol* the name of the protocol is given in 

quotes. 

* Finite sets are specified in braces 

through space- or comma-separated lists of  names. 

Names consist of alphanumerical characters or underscores. 

* The set of states is given after the identifier *states*. 

* In order to make transitions identifiable, 

each transition must be given a name after the  key word *transitions*.

* Transitions are defined after the key word *arcs*; there must be one definition

  for each transition name. 

* The set followed by the keyword *initial* specifies states that can 

belong to an initial population. 

* The sets followed by the keywords *true* and *false* identify the states 

that map to true and false, respectively.

Usage

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