Write a short incomplete README

Signed-off-by: Niels Thykier <niels@thykier.net>

README

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-README for britney v2.0
-=======================
-
-Please write here some useful stuff.
-

README.md

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+# Britney - Keeps suites installable and up to date
+
+Britney is a program to compute an update of a Debian-based package suite
+by feeding it updates from (one or more) source-suite(s).  A few known use
+cases:
+
+ * Debian uses it to update testing based on unstable
+ * Ubuntu uses it to update their latest development suite using a "hidden" -proposed suite as source
+
+Britney's primary goal is too keep packages in the target suite installable
+(e.g. Debian testing) while keeping it up to daye with its primary source
+suite (e.g. Debian unstable).
+
+## Quick setup guide
+
+TODO: Write a quick setup guide (once mirror layout has been tested)
+
+## Migration items
+
+Britney generally works with a "migration item", which is a group of binary
+packages (and possibly a source package).  Packages are bundled into these
+migration items under the following rules:
+
+ 1. "source migration": An update of the source package.  This will include all the binary packages built from that source version (regardless of architecture).
+   * Can contain binaries built from earlier source version depending on the setting of "IGNORE_CRUFT"
+   * Britney refers to these as "${SOURCE_NAME}"
+ 1. "binary migration": An update of binary packages on a given architecture to an existing source package in the target suite.
+   * Two common cases: Built for the first time on a new architecture and binNMUs
+   * Britney refers to all cases of these as "${SOURCE_NAME}/${ARCHITECTURE}"
+ 1. "removal item": A removal of a source or binary package.
+   * Note that it is only possible to trigger "source" removals via hints.  Binary removals are items generated by Britney to clean up the target suite.
+   * Britney refers to these as "-${SOURCE_NAME}" or "-${BINARY_NAME}/${ARCHITECTURE}" depending on the case.
+
+## Migration rules (excuses/policies)
+
+Britney applies a number of policies to migration items before attempting
+to migrate them to the target suite.  These policies can "reject" a
+package and prevent it from migrating.  Some policies/built-in rules:
+
+ * Age policy: Lets source migrations age a bit before they are allowed to migrate
+   - Supports variable length based on package urgency
+ * RC Bug policy: Rejects packages with regressions in RC bugs
+   - Requires an external tool to keep the bug lists up to date
+ * Keeps architectures in sync: Source migrations updating existing packages only occur if architectures are up to date
+   - Can be configured to ignore certain architectures.
+