Add prerequisite (#8089)

**Description**
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Please include a summary of the change and which issue is fixed. Please
also include relevant motivation and context. List any dependencies that
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Added prerequisite to have traefik installed as it's required for
Ingress.

⚒️ Fixes  # <!--(issue)-->

**⚙️ 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?**
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N/A

**📃 Notes:**
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Just helped a user setting up cert-manager and this situation arose.

**✔️ 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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Signed-off-by: Malcolm <56974882+malcolmcdixon@users.noreply.github.com>

charts/enterprise/cert-manager/docs/how-to.md

@@ -6,6 +6,8 @@ This guide will walk you through setting up `cert-manager`, certificate manageme
 
 Ensure you have the `enterprise` train enabled for `TrueCharts` as discussed [here](https://truecharts.org/manual/SCALE/guides/getting-started/#adding-truecharts).
 
+Ensure you have traefik installed, required for Ingress.
+
 Search for cert-manager in the `Apps` menu | `Available Applications` tab and click **Install**.
 
 ## Cloudflare DNS-Provider