Document ClassicAPI.dll requirement in README

Missing/incorrect ClassicAPI.dll causes errors and a blank grey readout; add setup and troubleshooting notes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -3,6 +3,14 @@
 A lightweight latency monitor for Vanilla WoW (1.12.1). Shows a small, movable,
 color-coded latency readout on your screen.
 
+### Requirements
+
+- **ClassicAPI.dll** must be installed before you install this addon. Without
+  it, the addon will throw errors and the readout will show as an empty grey
+  box with no text/numbers inside it.
+  - You'll need to add `ClassicAPI.dll` to your client's `dll.txt` (the entry
+    is case-sensitive).
+
 ### What it does
 
 - Displays your current world latency (falls back to home latency if world
@@ -39,3 +47,12 @@ you to report:
 
 The log is also saved to your `SavedVariables` file, so it survives
 logout/relog.
+
+### Troubleshooting
+
+- **Grey box with no numbers / constant errors**: This almost always means
+  `ClassicAPI.dll` isn't installed correctly. Check that:
+  - `ClassicAPI.dll` is present in your client folder.
+  - It's listed in `dll.txt` exactly as `ClassicAPI.dll` (matching case — the
+    entry is case-sensitive).
+  - After adding it, fully restart the client (not just `/reload`).