docs(filebrowser): additional storage note (#16567)

**Description**
Adds note about setting up additional app storage.

⚒️ Fixes  # 

**⚙️ Type of change**

- [ ] ⚙️ Feature/App addition
- [ ] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code

**🧪 How Has This Been Tested?**

**📃 Notes:**

**✔️ Checklist:**

- [ ] ⚖️ My code follows the style guidelines of this project
- [ ] 👀 I have performed a self-review of my own code
- [ ] #️⃣ I have commented my code, particularly in hard-to-understand
areas
- [ ] 📄 I have made corresponding changes to the documentation
- [ ] ⚠️ My changes generate no new warnings
- [ ] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
- [ ] ⬆️ I increased versions for any altered app according to semantic
versioning

**➕ App addition**

If this PR is an app addition please make sure you have done the
following.

- [ ] 🪞 I have opened a PR on
[truecharts/containers](https://github.com/truecharts/containers) adding
the container to TrueCharts mirror repo.
- [ ] 🖼️ I have added an icon in the Chart's root directory called
`icon.png`

---

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charts/stable/filebrowser/docs/installation_notes.md

@@ -12,3 +12,7 @@
 ## Storage Access
 
 - Mount Hostpath to `App Data Storage` and point it to the location you want to access from filebrowser
+
+## Additional App Storage
+
+- To add `Additional App Storage` add `/data` to the beginning of the `Mount Path`. For example, if you want to add a mount for `/photos` the `Mount Path` would be `/data/photos`.