doc: Refactor the contribution guidelines
doc: Refactor the contribution guidelines

Ensuring that each file changed by a patch has the correct copyright and
license information does not only apply to documentation files but to
all files within the source tree.

Move the guidance for copyright and license headers out of the paragraph
about updating the documentation to avoid any confusion.

Also do some cosmetic changes (adding empty lines, fitting in longer
lines in the 80-column limit, ...) to improve the readability of the RST
file.

Change-Id: I241a2089ca9db70f5a9f26b7070b947674b43265
Signed-off-by: Sandrine Bailleux <4b69fcfca8da45b0c11ecda5434c5da526575d9d@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware