Read and write to /etc/hosts

TL;DR:
- Rename website-blocker to url-blocker
- Add a README.md
- Reads and writes to /etc/hosts

tools/url-blocker/README.md

@@ -0,0 +1,47 @@
+# url-blocker
+
+`url-blocker` blocks the URLs that you want to block when you want it to block
+them.
+
+Let's say that you don't want to visit Twitter during the work week. Create the
+file `~/.config/url-blocker/rules.json` with the following contents and
+`url-blocker` will take care of the rest.
+
+```json
+# ~/.config/url-blocker/rules.json
+[
+  {
+    "urls": [
+      "twitter.com",
+      "www.twitter.com",
+    ],
+    "allowed": [
+      {
+        "day": "Saturday",
+        "timeslots": [
+          "00:00-11:59"
+        ]
+      },
+      {
+        "day": "Sunday",
+        "timeslots": [
+          "00:00-11:59"
+        ]
+      }
+    ]
+  }
+]
+```
+
+## Installation
+
+```shell
+$ nix-env -iA 'briefcase.tools.url-blocker'
+```
+
+## How does it work?
+
+`systemd` is intended to run `url-blocker` once every minute. `url-blocker` will
+read `/etc/hosts` and map the URLs defined in `rules.json` to `127.0.0.1` when
+you want them blocked. Because `systemd` run once every minute, `/etc/hosts`
+should be current to the minute as well.