Address Soha's feedback.

* Describe BUILD_ variables.
* Discuss CMAKE_BUILD_TYPE, and building in both Debug and Release modes.
* Mention `make doxygen` for UNIX.
Touch up formatting.
Merge pull request #147 from chrisdembia/readme-Soha-comments

Address Soha's feedback.
Address order of build types.
Fix language about release build type.
Merge pull request #148 from chrisdembia/readme-debug-release-order

Readme debug release order
Fixes #149. Change LIBDIR if installing to /usr/local.

Made corresponding changes to the build/install instructions.
Elaborate on installation layout.
Incorporate @j-rivero's feedback.
Merge pull request #151 from chrisdembia/usrlocal

Thanks, Chris and Jose. Fixes #149. Change LIBDIR if installing to /usr/local.
Incorporate some content from old instructions.
Merge pull request #152 from chrisdembia/patch-1

Incorporate some content from old instructions.
Relative link for RNA gif.
Merge pull request #153 from chrisdembia/patch-5

Relative link for RNA gif.
Merge branch 'master' of https://github.com/simbody/simbody into readmeGIF
Changed doublePendulum.gif path to relative.