chore: add README for topic heartbeat example

Adds a README for the new topic heartbeat example.

examples/TopicHeartbeatExample/README.md

@@ -0,0 +1,25 @@
+# Topic Example
+
+This example program demonstrates how to use a [Timer](https://learn.microsoft.com/en-us/dotnet/api/system.timers.timer?view=net-8.0) to ensure that a heartbeat is received from a topic within a certain time frame.
+
+# Usage
+
+The program assumes the auth token and cache names are available in environment variables. The auth token is assumed to be in the variable `MOMENTO_API_KEY` and the cache name in `MOMENTO_CACHE_NAME`. If either of these is missing, you will be prompted to enter the values on the terminal.
+
+To run the program, run either:
+
+```bash
+MOMENTO_API_KEY=<YOUR_API_KEY_HERE> MOMENTO_CACHE_NAME=<YOUR_CACHE_NAME_HERE> dotnet run
+```
+
+or
+
+```bash
+dotnet run
+```
+
+and you will be prompted to enter the auth token and cache name.
+
+If the cache name entered does not exist, the program will create it.
+
+The example publishes one message per second to a topic for 30 seconds. It subscribes to the same topic and prints each message it receives.