Move documentation into README and doc/

* Moves general documentation, such as the description and installation
  instructions, to a new README file at the root of the repo.
* Moves detailed documentation, such at command descriptions, into a text file
  under doc/ that can be indexed and searched through using Vim's `:help` command.

README.markdown

@@ -0,0 +1,82 @@
+# vim-gnupg
+
+This script implements transparent editing of gpg encrypted files. The filename
+must have a `.gpg`, `.pgp` or `.asc` suffix. When opening such a file the
+content is decrypted, when opening a new file the script will ask for the
+recipients of the encrypted file. The file content will be encrypted to all
+recipients before it is written. The script turns off viminfo, swapfile, and
+undofile to increase security.
+
+## Installation
+
+Copy the `gnupg.vim` file to the `$HOME/.vim/plugin` directory. Refer to `:help
+add-plugin`, `:help add-global-plugin` and `:help runtimepath` for more details
+about Vim plugins.
+
+From `man 1 gpg-agent`:
+
+> You should always add the following lines to your `.bashrc` or whatever
+> initialization file is used for all shell invocations:
+>
+>     GPG_TTY=`tty`
+>     export GPG_TTY
+>
+> It is important that this environment variable always reflects the output of
+> the tty command. For W32 systems this option is not required.
+
+Most distributions provide software to ease handling of gpg and gpg-agent.
+Examples are keychain or seahorse.
+
+If there are specific actions that should take place when editing a
+GnuPG-managed buffer, an autocmd for the User event and GnuPG pattern can be
+defined. For example, the following will set `textwidth` to 72 for all
+GnuPG-encrypted buffers:
+
+    autocmd User GnuPG setl textwidth=72
+
+This will be triggered before any BufRead or BufNewFile autocmds, and therefore
+will not take precedence over settings specific to any filetype that may get
+set.
+
+## Known Issues
+
+In some cases gvim can't decrypt files.
+
+This is caused by the fact that a running gvim has no TTY and thus gpg is not
+able to ask for the passphrase by itself. This is a problem for Windows and
+Linux versions of gvim and could not be solved unless a "terminal emulation" is
+implemented for gvim. To circumvent this you have to use any combination of
+gpg-agent and a graphical pinentry program:
+
+- gpg-agent only:
+  you need to provide the passphrase for the needed key to gpg-agent
+  in a terminal before you open files with gvim which require this key.
+- pinentry only:
+  you will get a popup window every time you open a file that needs to
+  be decrypted.
+- gpgagent and pinentry:
+  you will get a popup window the first time you open a file that
+  needs to be decrypted.
+
+## Credits
+
+- Mathieu Clabaut for inspirations through his vimspell.vim script.
+- Richard Bronosky for patch to enable `.pgp` suffix.
+- Erik Remmelzwaal for patch to enable windows support and patient beta testing.
+- Lars Becker for patch to make gpg2 working.
+- Thomas Arendsen Hein for patch to convert encoding of gpg output.
+- Karl-Heinz Ruskowski for patch to fix unknown recipients and trust model and
+  patient beta testing.
+- Giel van Schijndel for patch to get `GPG_TTY` dynamically.
+- Sebastian Luettich for patch to fix issue with symmetric encryption an set
+  recipients.
+- Tim Swast for patch to generate signed files.
+- James Vega for patches for better `*.asc` handling, better filename escaping
+  and better handling of multiple keyrings.
+
+## License
+
+This program is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free Software
+Foundation; either version 2 of the License, or (at your option) any later
+version. See http://www.gnu.org/copyleft/gpl-2.0.txt