Add use info to README.md

Usage

-------

Peregrine is run from the command line. 

If you save the protocol from above in the file *majority.pp* and enter:

```bash

    peregrine majority.pp

```

You obtain the following output:

```

Peregrine - Efficient Verification of Population Protocols

Reading "majority.pp"

Analyzing Population protocol "Majority Protocol"

Checking layered termination

Checking layered termination with at most 1 layers

Checking layered termination with at most 2 layers

layered termination satisfied

Checking strong consensus

strong consensus satisfied

All properties satisfied

```

As seen in the output, Peregrine by default checks two properties: 

*Layered Termination* and *Strong Consensus*:

* Layered Termination: every fair run of the protocol

will eventually reach a terminal population where no more change occurs.

* Strong Consensus: every terminal population reachable from an initial population

  forms a consensus. Moreover, the boolean value of the consensus must be unique 

  for a given initial population.

If both Layered Termination and Strong Consensus are satsified, then the protocol

is *well-specified*, i.e. the protocol robustly computes a predicate defined

by the mapping from input population to its unique consensus value.

Note that the converse direction need not hold: there are well-specified protocols

that violate the conjunction of Strong Consensus and Layered Termination.

Peregrine offers fine-grained control over the solution methods and the 

verbosity of the output.

To see all options, enter:

```bash

    peregrine --help

```

Authors

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