A first/second tutorial

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/159)) by stonier-tri (Bitbucket: [stonier-tri](https://bitbucket.org/%7Bdf147a71-ec31-411c-b3b7-3bace0b40d64%7D/)).**

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As discussed with @caguero - it would be great to have a couple of introductory tutorials on the [web site](http://sdformat.org/tutorials) that demonstrate how to build an sdf and then parse it with the c++ api.