Copious comments for hook_plot_tex

Add comments to `hook_plot_tex` in `hooks-latex.R`, to make the function
easier to skim and debug. Maintain original coding style, of course.
Add 'Contributing' section to README.md

Add a section to the readme that describes how to
* get the code (with git and github)
* make changes and test them (with devtools), and
* make a pull request (with git and github).
Add trailing newline to render_html output block

Fix for issue #556
Merge pull request #557 from jimhester/render_html_newline

Add trailing newline to render_html output block; fixes #556
Merge pull request #552 from sietse/desc-development-in-readme

Add 'Contributing' section to README.md
Merge pull request #551 from sietse/comment-hook_plot_tex

Copious comments for hook_plot_tex
use the CRAN repository and the PPA c2d4u
the binaries of xml, rcpp, rgl and rcurl are available in c2d4u
a new function kable(); closes #53 since it's unlikely for me to work on it
a vignette example for the DataTables library
string `base` too wide; remove it and make it a constant

just for the aggressive R CMD check for "width>90"!
move the knit_expand() examples to a vignette

just for the aggressive R CMD check for "width>90"!
tweak the read_chunk() example to make it easier to break for formatR
roxygenize
question mark for #554
news for #555
white space
little r
ctb instead of crb for #555
add Nick Salkowski and Sietse Brouwer to the contributor list