Update README with web interface information

Added a section about the web interface and usage instructions.

Author: tstromberg

README.md

@@ -50,6 +50,10 @@ $ prcost https://github.com/chainguard-dev/apko/pull/1860
 
 ```
 
+## Web Interface
+
+Try https://cost.github.codegroove.app/ - it only has access to public repositories, so if you need to take that account for accuracy, run prcost locally.
+
 ## Caveats
 
 * Due to limited input, results may not be accurate for single PRs, but should be accurate on average across a larger sample size of PRs
@@ -98,6 +102,12 @@ prcost --org chainguard-dev
 prcost --org myorg --samples 100 --days 60
 ```
 
+### Web Interface
+
+```bash
+go run ./cmd/server
+```
+
 ### Sampling Strategy
 
 Repository and organization modes use time-bucket sampling to ensure even distribution across the time period. This provides more representative estimates than random sampling by avoiding temporal clustering.