Tutorial's install instructions still on version 2

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/133)) by Louise Poubel (Bitbucket: [chapulina](https://bitbucket.org/%7B5cfa2075-477b-4ded-bdb9-8d2479544ec4%7D/), GitHub: [chapulina](https://github.com/chapulina)).**

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Here:

http://sdformat.org/tutorials?tut=install&cat=get_started

Also, the install instructions in the [README](https://github.com/osrf/sdformat/blob/eebd329e9c9060a7fa889995eeb552f8efb4fc55/README) are incomplete, since they don't include dependencies. It might be a good idea to change the README to be like [Gazebo's](https://bitbucket.org/osrf/gazebo/src/75c110c1ba6ea5e103d731a50238999a6287984c/README?at=default&fileviewer=file-view-default), which links to the install tutorial.

Start with the install tutorial at http://sdformat.org/tutorials?tut=install&cat=get_started and the repository README mentioned in the report. Compare their instructions with the current version and dependency requirements, then update the tutorial and clarify the README's link or installation guidance so both are current and complete.

Written by the indexing model from the issue text.