add apk-world(8)

doc/Makefile

+$(obj)/%.5: $(src)/%.5.scd
+	$(SCDOC) < $< > $@
+
 $(obj)/%.8: $(src)/%.8.scd
 	$(SCDOC) < $< > $@
 
-docs: $(obj)/apk.8
+docs: $(obj)/apk.8 \
+	$(obj)/apk-world.5
 targets += docs
 
 install: docs

doc/apk-world.5.scd

+apk-world(5)
+
+# NAME
+
+*/etc/apk/world* - list of explicitly installed packages
+
+# DESCRIPTION
+
+At /etc/apk/world, apk maintains the _world_, or list of explicitly installed
+packages. This is a plaintext file with one package spec per line.
+
+If you edit this file manually, you should run *apk-fix*(8) to apply the
+changes.
+
+# PACKAGE SPECIFICATION
+
+Specifying a package name with no other modifiers will install the latest
+version of that package from the first repository in which it is available.
+
+To pin a package to a tagged repository, use the format *pkgname@tagname*, e.g.
+*busybox@edge*. See *apk-repositories*(5) for details on tagged package
+repositories.
+
+To constrain the acceptable versions of the package, use the *=*, *<*, *>*,
+*>=*, or *~=* operators. Respectively, these require the package is equal to,
+less than, greater than, or greater than or equal to the specified version. The
+*~=* operator constrains the package to the version numbers specified, but will
+not constrain any unspecified version numbers.
+
+*busybox*
+	Installs busybox from the first repository from which it is available.
+
+*busybox@edge*
+	Installs busybox from the first repository tagged "edge" from which
+	it's available. If busybox is _not_ available in repositories with this
+	tag, it will cause an error. When resolving dependencies of tagged
+	requirements, untagged repositories are preferred, but repositories
+	sharing the same tag will be considered if necessary.
+
+*busybox=1.6.1*
+	Install busybox version 1.6.1.
+
+*busybox>1.6.1*
+	Install a busybox version greater than 1.6.1.
+
+*busybox>1.6.1*
+	Install a busybox version greater than or equal to 1.6.1.
+
+*busybox<1.6.1*
+	Install a busybox version less than 1.6.1.
+
+*busybox~=1.6*
+	Install any busybox version between 1.6.0 (inclusive) and 1.7.0
+	(exclusive).
+
+# AUTHORS
+
+Natanael Copa <ncopa@alpinelinux.org>++
+Timo Teräs <_timo.teras@iki.fi_>