Adding how to run tests on developer notes.

[raphaeltimbo]

This adds an small paragraph for details on how to run the doctests.

[josephcslater]

Raphael- do you know what's going on with the failed checks? My merge from Master were typos, not code, that I didn't want clashes with. The travis log says it's the doc string in euler beam... I didn't know anyone was touching that.

[raphaeltimbo]

@josephcslater I was taking a look at this, and I think that the revert from #41 did not happen, since it was not merged (if we look at the master branch the code from #39 is there).
I saw the errors and they were due to a print statement and 2 figures that the output was not present at the docstrings.
I think we can merge this pull request and close #41 , this way we can use the code that has already been written by @AnushaAnisetti on #39 .

[josephcslater]

Thank you for figuring this out. I expected to spend time this weekend on it.