Merge pull request #2 from wintoncode/readme

Added README

README.md

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+# Winton.DomainModelling.Abstractions
+
+[![Build status](https://ci.appveyor.com/api/projects/status/7mba8m947ed603r1?svg=true)](https://ci.appveyor.com/project/wintoncode/winton-domainmodelling-abstractions/branch/master)
+[![Travis Build Status](https://travis-ci.org/wintoncode/Winton.DomainModelling.Abstractions.svg?branch=master)](https://travis-ci.org/wintoncode/Winton.DomainModelling.Abstractions)
+[![NuGet version](https://img.shields.io/nuget/v/Winton.DomainModelling.Abstractions.svg)](https://www.nuget.org/packages/Winton.DomainModelling.Abstractions)
+[![NuGet version](https://img.shields.io/nuget/vpre/Winton.DomainModelling.Abstractions.svg)](https://www.nuget.org/packages/Winton.DomainModelling.Abstractions)
+
+Abstractions useful for modelling a domain.
+
+## Building blocks
+
+### Entity
+
+A base class to implement entity types, which are defined by their identity rather than their attributes. Implementers are equal if their IDs are equal. Any equatable ID type can be used.
+
+## Exceptions
+
+### DomainException
+
+Represents domain errors. Extensible for any domain-specific error.
+
+### EntityNotFoundException
+
+Extends `DomainException` to indicate that an entity could not be found.
+
+### UnauthorizedException
+
+Extends `DomainException` to indicate that the action being performed is not authorized.