git: Added pcap plugin to plugin list (WerWolv#1395)

Adds a reference to [Pcap
Plugin](https://github.com/Professor-plum/ImHex-Plugin-Pcap) under
Third-Party Plugins

### Problem description
Adds a reference to an external repository for an ImHex Plugin to view
raw data within pcap files.

### Implementation description
The ImHex Plugin is implemented as an additional Data Provider. It
allows BPFs and selecting multiple packets for data reassembly.

### Screenshots

![screenshot](https://raw.githubusercontent.com/Professor-plum/ImHex-Plugin-Pcap/master/Images/Screenshot.png)

### Additional things
Should there be any questions I'm also available on Discord.

PLUGINS.md

@@ -15,3 +15,5 @@ To install plugins, simply download the relevant `.hexplug` file and drop it in
   - Adds support for Discord Rich Presence
 
 ### Third-Party Plugins
+- [Pcap Plugin](https://github.com/Professor-plum/ImHex-Plugin-Pcap)
+  - Adds support for reading packet capture files