Added a library usage example in README.

README.md

@@ -96,3 +96,37 @@ And if answer was correct a client gets a HTTP code 202. Or 403 otherwise.
 Then a server checks if CAPTCHA was solved with following request.
 
     GET /n60f2K9JiD5c4qX9MYe90A54nT0nnJrtgfhAjfaWtBg
+
+## Library usage
+
+A simple example using built-in dwelling CAPTCHA implementation.
+
+Create a new CAPTCHA:
+
+    c := dwcaptcha.NewDwellingCaptcha(expiry)
+    _, id := inmemdb.New(someAdditionalDataUsedInIDGenerationUsuallyIPAddr, c)
+
+Get an image for a CAPTCHA:
+
+    i := inmemdb.Image(captchaID, captchaStyle)
+    if i == nil {
+        ... // error handling
+    }
+
+    jpeg.Encode(w, *i, &jpeg.Options{Quality: 20})
+
+Solve a CAPTCHA:
+
+    if ok := inmemdb.Solve(captchaID, answer); !ok {
+        ... // not solved
+    }
+    // solved
+    ...
+
+Check is CAPTCHA was solved:
+
+    if ok := inmemdb.IsSolved(captchaID); !ok {
+	    ... // not solved
+    }
+    // solved
+    ...