hard code path of README.md (#3035)

### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
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* [x] I have added Go unit tests or [Complement integration
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_or_ I have already signed off privately

I was reading on
https://matrix-org.github.io/dendrite/faq#is-dendrite-feature-complete.
The link is dead due to relative path.

🍻

Signed-off-by: `Aiden Leong <[email protected]>`

Author: aiden-leong

docs/FAQ.md

@@ -18,7 +18,7 @@ Mostly, although there are still bugs and missing features. If you are a confide
 
 ## Is Dendrite feature-complete?
 
-No, although a good portion of the Matrix specification has been implemented. Mostly missing are client features - see the [readme](../README.md) at the root of the repository for more information.
+No, although a good portion of the Matrix specification has been implemented. Mostly missing are client features - see the [readme](https://github.com/matrix-org/dendrite/blob/main/README.md) at the root of the repository for more information.
 
 ## Why doesn't Dendrite have "x" yet?