Manual.md: designate place for Void-specific documentation

[Chocimier]

This is meant to be used instead of install.msg for not critical information, and maybe some information from wiki.

I will clean up install.msgs if this is accepted.

[ericonr]

Huh, this is great! I will wait for it to be merged, then I can make a PR for the info at void-docs.

[CameronNemo]

Which wiki information? Which INSTALL.msgs might be replaced by this?

Note, here is a current list of all install messages that are not the "removed package" string:

http://ix.io/2iLy

I feel like the apparmor, bluez/bluez-qt5/blueman, wicd, socklog-void messages could be moved to the official docs.

For rng-tools, "Note that version 6 of rng-tools has modified the rngd options" -- version 6 was released in July of 2017! Maybe time to remove that note...

I implore contributors to:

1. clean up what we have in the official docs and INSTALL.msg files
2. take stock of which information from the wiki is important and needs to be migrated

[CameronNemo]

I guess optional dependencies could be documented there, but I am not sure how many people will look... python3-matplotlib and scapy both use INSTALL.msg files to suggest optional dependencies.

Likewise service configuration that is less-than-straightforward (e.g. mopidy, minio) seems like a good category of information to put in these README files.

[Chocimier]

Install.msg is meant for most important messages like possible data loss or being locked out (openssh) or so. Optional dependencies will stay here for now, but there is proposal that they will be moved to xbps-query output.

I want to move other messages that are not trivial (dhewm3 is, for example), into readmes.

For wiki, i won't move anything, just creating space for content that was asked to move to docs, but do not fit there.