Add usage / example

Signed-off-by: Knut Ahlers <knut@ahlers.me>

README.md

@@ -10,3 +10,38 @@
 - HTTP status code 2xx
 - Regular expression match on the response body
 - Request answer within certain timeout
+
+## Usage
+
+```console
+$ webcheck --help
+Usage of webcheck:
+  -i, --interval duration        Check interval (default 1s)
+  -l, --log-dir string           Directory to log non-matched requests to (default "./request-log/")
+      --log-retention duration   When to clean up file from log-dir (default 24h0m0s)
+  -m, --match string             RegExp to match the response body against to validate it (default ".*")
+      --no-log                   Disable response body logging
+  -t, --timeout duration         Timeout for the request (default 30s)
+  -u, --url string               URL to query
+      --version                  Prints current version and exits
+```
+
+### Example
+
+```console
+$ webcheck -u https://bfa1c797.eu.ngrok.io/monitoring.txt -m healthy
+
+[Mon, 23 Jul 2018 16:07:02 CEST] (OKAY) Status was 200 and text matched (13.331ms/14.229ms/115.599ms)
+[Mon, 23 Jul 2018 16:07:16 CEST] (FAIL) Response body does not match regexp (13.314ms/14.229ms/15.316ms) (Resp: request-log/request827008143)
+[Mon, 23 Jul 2018 16:07:21 CEST] (OKAY) Status was 200 and text matched (13.411ms/14.436ms/18.25ms)
+[Mon, 23 Jul 2018 16:07:28 CEST] (FAIL) Status code was != 2xx: 404 (6.923ms/7.011ms/7.237ms) (Resp: request-log/request070057634)
+
+$ cat request-log/request827008143
+Accept-Ranges: bytes
+Content-Length: 4
+Content-Type: text/plain; charset=utf-8
+Date: Mon, 23 Jul 2018 14:07:16 GMT
+Last-Modified: Mon, 23 Jul 2018 14:07:15 GMT
+
+foo
+```