Add code climate badge

From https://codeclimate.com/docs
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Quality Metrics

What is a Smell?

Code Smells (or simply "Smells") are symptoms of your code that
possibly indicate a deeper problem. Code Climate identifies
long methods and duplicated blocks of code as smells and soon
will also identify other smells.

Smells are not necessarily problems themselves. For example, long
methods are often a symptom of mismanaged object responsibilities
that require changes to the domain model. Simply splitting up
the long method into smaller methods is not always the way to go.

It's important to note that not all smells identified by Code
Climate point to issues that need to be addressed. Programmer
judgment must be applied when using code metrics.

What is the Complexity metric?

Complexity metrics are based on Flog, an open source Ruby
static analysis tool by Seattle.rb. The Complexity metric
is a variant of the ABC metric, which looks at Assignments,
Branches and Conditions in code. Certain Ruby metaprogramming
constructs are also considered, as they generally make code
harder to understand quickly.

What is the Duplication metric?

Duplication metrics are based on Flay, another great open source
Ruby static analysis tool created by Seattle.rb. Code Climate
looks at the syntax trees of your Ruby code in order to identify
identical and similar code structures. Because duplication is
identified using syntax trees, it's resilient to being confused
by changes in code formatting and simple renames.

What is the Churn metric?

Churn is simply the number of times the source code file has
changed in your Git history. Classes that change frequently and
are also complex and difficult to maintain are good targets
for refactoring. Writing small classes that have a single
responsibility, and are open for extension but closed for
modification helps avoid churn.

Signed-off-by: Paul Morgan <jumanjiman@gmail.com>