Added a contributing file

More discoverable then our documentation on contributing http://nest.azurewebsites.net/contributing.html

contributing.md

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+#Sign the CLA
+
+Contributing back to `Elasticsearch.Net` and `NEST` is very much appreciated. 
+Whether you [feel the need to change one character](https://github.com/elasticsearch/elasticsearch-net/pull/536) or have a go at 
+[mapping new API's](http://github.com/elasticsearch/elasticsearch-net/pull/376) no PR is too small or too big. 
+
+In fact many of our most aweseome features/fixes have been provided to us by [these wonderful folks](https://github.com/elasticsearch/elasticsearch-net/graphs/contributors) to which we are forever indebted 
+
+We do however ask you sign the [Elasticsearch CLA](http://www.elasticsearch.org/contributor-agreement/) before we can accept pull requests from you. 
+
+## Coding Styleguide
+
+Please install the [editorconfig vs extension](https://visualstudiogallery.msdn.microsoft.com/c8bccfe2-650c-4b42-bc5c-845e21f96328)
+this will automatically switch to our indentation, whitespace, newlines settings while working on our project
+**while leaving your default settings intact**.
+
+In most cases we won't shun a PR just because it uses the wrong indentation settings though it'll be **very** much appreciated!
+
+## Tests
+
+PR's with tests are more likely to be reviewed faster because it makes our jobs that much more easy, that being said
+we respect that you are fixing a bug in your own time and might not have the time/energy to submit a PR with complete tests. 
+In those cases we tend to pull your bits locally and write tests ourselves but this may mean your PR might sit idle longer then you would like.
+
+## Git
+
+We do not require rebased/squashed commits although we do very much appreciate it! 
+
+Please submit your PR's to our develop branch.
+
+