Update Package-development-cycle.rmd (typos/suggestions)

* Removed extra "already" from line 8.
* Changed "supported" to "support" on line 61
* Changed "Rstudio" to "RStudio" on line 61, to be consistent with how it is styled on RStudio.com =)
* Also on line 61: confirmed ctrl+shift+L is still correct for RStudio. Also appears that as of [ESS 13.09](http://ess.r-project.org/Manual/ess.html), `C-c C-t l` in ESS will run `load_all`. I did not add the ESS change in the source, since you just have "ESS?" written there. Just wanted to point this out in case you did want to add it.
* Changed "Repeat 2--4" to "Repeat 2--3" on line 80, in order to be consistent with the instruction style of line 96.
* Added a Wikipedia link for "extreme programming". Even though it is linked in the test-driven development Wikipedia entry, I thought it might be useful and more direct for readers. Lines 71 and 106.
Merge pull request #318 from liangcj/patch-4

Update Package-development-cycle.rmd (typos/suggestions)