doc/README.rst: add reStructuredText README

Mainly for the benefit of Codeberg, but having something in plain text
is nice too I guess.

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+Overview
+========
+
+*This is only a brief introduction, see the man page for details.*
+
+Hath is a CLI utility for working with network blocks in CIDR
+notation. There are a few things that one usually wants to do with
+said blocks. Hath takes a list of CIDR blocks as input (via stdin),
+and does one of the following, depending on the mode:
+
+regexed mode (default)
+  Create a regular expression matching the given blocks.
+reduced mode
+  Combine consecutive small blocks into larger ones.
+duped mode
+  List the blocks that are redundant or could be combined
+  into larger blocks.
+diffed mode
+  Similar to reduced mode, but showing the changes in a diff-like
+  format.
+listed mode
+  List the addresses in the blocks sequentially.
+
+
+Usage
+=====
+
+.. code-block:: console
+
+  $ hath [regexed|reduced|duped|diffed|listed] [-hb] <input>
+
+
+Examples
+========
+
+.. code-block:: console
+
+  $ echo "10.0.0.0/29 10.0.0.8/29" | hath
+  ((10).(0).(0).(15|14|13|12|11|10|9|8|7|6|5|4|3|2|1|0))
+
+.. code-block:: console
+
+  $ echo "10.0.0.0/24 10.0.1.0/24" | hath reduced
+  10.0.0.0/23
+
+.. code-block:: console
+
+  $ echo "10.0.0.0/24 10.0.1.0/24" | hath duped
+  10.0.0.0/24
+  10.0.1.0/24
+
+.. code-block:: console
+
+  $ echo "10.0.0.0/24 10.0.1.0/24" | hath diffed
+  -10.0.0.0/24
+  -10.0.1.0/24
+  +10.0.0.0/23
+
+.. code-block:: console
+
+  $ echo 192.168.0.240/29 | hath listed
+  192.168.0.240
+  192.168.0.241
+  192.168.0.242
+  192.168.0.243
+  192.168.0.244
+  192.168.0.245
+  192.168.0.246
+  192.168.0.247
+
+
+Requirements
+============
+
+All dependencies are listed in the ``hath.cabal`` file. Just use cabal
+to build it.
+
+
+Installation
+============
+
+Cabal handles the build, so do whatever you normally do to install
+cabal packages. If you just want to install it for your user,
+
+.. code-block:: console
+
+  user $ runghc Setup.hs configure --user
+  user $ runghc Setup.hs build
+  user $ runghc Setup.hs install
+
+should do it.
+
+
+Bugs
+====
+
+Email them to me at michael@orlitzky.com, or report them on `Codeberg
+<https://codeberg.org/mjo/hath/issues>`_