Tidy up structure of briefcase

I had a spare fifteen minutes and decided that I should tidy up my
monorepo. The work of tidying up is not finished; this is a small step in the
right direction.

TL;DR
- Created a tools directory
- Created a scratch directory (see README.md for more information)
- Added README.md to third_party
- Renamed delete_dotfile_symlinks -> symlinkManager
- Packaged symlinkManager as an executable symlink-mgr using buildGo

tools/rfcToKindle/README.md

@@ -0,0 +1,30 @@
+# rfcToKindle
+
+Wirelessly transfer RFC documents to your Kindle to device for an alternative
+medium for reading.
+
+## Installation
+
+`rfcToKindle` makes use of [`buildGo.nix`][2] to package itself.  If you're
+using [Nix][1], you can install `rfcToKindle` using `nix-env`:
+
+```shell
+> nix-env -f https://github.com/wpcarro/rfcToKindle -i
+```
+
+## Usage
+
+```shell
+> rfcToKindle -document rfc6479 -recipient username@kindle.com
+```
+
+## Dependencies
+
+This uses `sendgmr` to send the file to the Kindle. Make sure:
+1. That `sendgmr` is installed and available on $PATH.
+2. That it is configured to work with your preferred email address.
+3. That the email address `sendgmr` is configured to use is whitelisted in
+   your Kindle "Personal Document Settings".
+
+[1]: https://nixos.org/nix/
+[2]: https://git.tazj.in/tree/nix/buildGo