doc: Adjust "Building from Git" instructions.

* HACKING (Building from Git): As a follow-up to ff1cbb9, update the
  error reported when pkg.m4 is missing.  Suggested by
  John Darrington <john@darrington.wattle.id.au>.

Merge branch 'master' into core-updates

doc: Add "Data Types and Pattern Matching" to the Coding Style.

* HACKING (Data Types and Pattern Matching): New section.

doc: Add note about .dir-locals.el.

* HACKING (Coding Style): Mention special indentation rules and
  .dir-locals.el.

doc: Add a "Coding Style" section in 'HACKING'.

* HACKING (Coding Style): New section.

doc: Remove "Adding new packages" from `HACKING'.

* HACKING (Adding new packages): Remove since it's now in the manual.

doc: Change `HACKING' to refer to guix-devel@gnu.org.

Reported by Alex Sassmannshausen <alex.sassmannshausen@gmail.com>.

* HACKING: Replace "bug-guix" by "guix-devel".

doc: Improve build instructions in README and HACKING.

* README (Requirements): Remove Nixpkgs.  Remove mentions of building
  from Git.
  (Installation): Refer to the manual.
* HACKING: List requirements for when building from Git.
  Remove the `dot: Command not found' error.

doc: Add "Building from Git" to 'HACKING'.

Merge branch 'core-updates'

doc: Move the packaging guidelines to the manual.

* HACKING (Packaging Guidelines): Remove.
* doc/guix.texi (Packaging Guidelines): New node.

doc: Add a "Porting" section.

* HACKING (Porting the Guix distro on a new platform): Remove.
* doc/guix.texi (Porting): New node.  Describe cross-compilation as the
  only approach.

doc: Mention copyright/license auditing in `HACKING'.

doc: Update bootstrap-related info in `HACKING'.

doc: Write about patch submission and packaging guidelines.

* HACKING: Update the command names from `guix-build' to `guix build' & co.
  (Submitting Patches, Packaging Guidelines): New sections.
* doc/guix.texi (Contributing): New section.

doc: Add the commit policy to HACKING.

* HACKING (Commit Access): New section.

doc: Add "The Perfect Setup" in 'HACKING'.

* HACKING (The Perfect Setup): New section.

Update 'HACKING'.

* HACKING (When the platform is supported by Nixpkgs): Update the example.

Update `HACKING'.

* HACKING: Capitalize the title.
  (Running Guix before it is installed): New section.
  (Adding new packages): Update example.

Merge branch 'core-updates'

Conflicts:
	guix/build/union.scm