Update the README.

A quick start example, licence information, etc.

Author: hainesr

README.md

@@ -6,3 +6,37 @@ A Ruby library for manipulating CITATION.cff files.
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+
+### Synopsis
+
+This library provides a Ruby interface to manipulate CITATION.cff files. The primary entry points are the Model and File classes.
+
+See the [CITATION.cff documentation](https://citation-file-format.github.io/) for more details.
+
+### Quick start
+
+```ruby
+cff = CFF::Model.new("Ruby CFF Library")
+cff.version = 0.1.0
+cff.date_released = Date.today
+cff.authors << CFF::Person.new("Robert", "Haines")
+
+CFF::File.write("CITATION.cff", cff)
+```
+
+Will produce a file that looks something like this:
+
+```
+cff-version: 1.0.3
+message: If you use this software in your work, please cite it using the following metadata
+title: Ruby CFF Library
+version: 0.1.0
+date-released: 2018-02-18
+authors:
+- family-names: Haines
+  given-names: Robert
+```
+
+### Licence
+
+[Apache 2.0](http://www.apache.org/licenses/). See LICENCE for details.