[gentoo-dev] readme.gentoo.eclass: Add a DISABLE_AUTOFORMATTING variable

[Pacho Ramos]

Currently, when people uses DOC_CONTENTS variable to place their desired
messages, they are automatically reformatted by "fmt" to get proper
messages (for example, splitting long lines).

But, in some cases, may be useful to disable this behavior and respect
strictly how DOC_CONTENTS was formatted, for example in that kind of
messages telling people to run a command and, then, requiring a new line
to be used. This can also be useful to append extra information to
DOC_CONTENTS when, for example, additional info is needed when enabling
a USE flag.