Doc: Improve Installation section

- Add a subsection title 'Linux, MacOS, * BSD' for clarity
  - Add a link to the documentation of Firefox on NativeMessaging
  - Add both manifest locations on OSX: Per-user & System-wide

README.md

@@ -19,6 +19,9 @@ This piece of software wraps around the **[zx2c4 pass](http://www.zx2c4.com/proj
 * `sed`
 
 ### Installation
+
+#### Linux, MacOS, * BSD
+
 Download the `install_host_app.sh` script from [our releases page](https://github.com/passff/passff-host/releases) and execute it. You can do this in one line like so:
 
 ```
@@ -54,12 +57,13 @@ This will copy the host application and manifest files to the right place for yo
 
 If this doesn't work, you can link the files yourself. First, change the `path` value in the `passff.json` file to be the absolute path to the project's `bin/testing/passff.py` file. Then symlink (or copy) the file `bin/testing/passff.json` to the appropriate location for your browser and OS:
 
-- Firefox
+- [Firefox](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Native_manifests#Manifest_location)
   - Linux
     - Per-user: `~/.mozilla/native-messaging-hosts/passff.json`
     - System-wide: `/usr/{lib,lib64,share}/mozilla/native-messaging-hosts/passff.json`
   - OS X
-    - `/Library/Application Support/Mozilla/NativeMessagingHosts/passff.json`
+    - Per-user: `~/Library/Application Support/Mozilla/NativeMessagingHosts/passff.json`
+    - System-wide: `/Library/Application Support/Mozilla/NativeMessagingHosts/passff.json`
   - Windows
     - Per-user: `Path contained in registry key HKEY_CURRENT_USER\Software\Mozilla\NativeMessagingHosts\passff`
     - System-wide: `Path contained in registry key HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\NativeMessagingHosts\passff`