Add README with a quick guide

README.md

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+# Quick guide to ahtiports
+
+`/etc/apk/repositories` points to a local index of packages at
+`/var/ahtiports/packages/ports`. Packages we are maintaining ourselves are
+pinned to that repository, meaning that no other updates on other repositories
+are considered.
+
+The `ahtiports` user may build new packages based on the information in this
+repository. Whilst the `ahtiports` user may also be used to test and commit
+changes to this repository, it is recommended to test the build with your local
+user such that the pristine package index is not disturbed until we are sure
+that a package actually works.
+
+## Prerequisites
+
+Your user needs to be in the `abuild` group.
+
+## Updating a package
+
+First, clone this repo on any Alpine system (or ahti, of course). For a general
+overview on Alpine packaging, see
+[Creating an Alpine package](https://wiki.alpinelinux.org/wiki/Creating_an_Alpine_package).
+
+Make your changes to the package in question, then create the checksums and build the package:
+
+```
+    $ abuild checksum
+    $ abuild -r
+```
+
+Your package will be placed into `~/packages`. If the build succeeded, you may
+clean any temporary files using `abuild clean` and then commit your changes and
+push them to this repository. You may also try using the `abump` tool.
+
+## Placing a package in the local index
+
+Switch to the `ahtiports` user:
+
+```
+    $ sudo -iu ahtiports
+```
+
+This repository is available in `~/ports`. Pull your changes and then build the
+package with `abuild -r`. The package is now available in
+`/var/ahtiports/packages/ports` and will be visible to `apk`. A normal system
+upgrade will pick it up.