[readme] add doc for --no-only flag

readme.markdown

@@ -144,6 +144,23 @@ By default, uncaught exceptions in your tests will not be intercepted, and will
 - In-process reporting with https://github.com/DavidAnson/tape-player
 - Describe blocks with https://github.com/mattriley/tape-describe
 
+# command-line flags
+
+While running tests, top-level configurations can be passed via the command line to specify desired behavior.
+
+Available configurations are listed below:
+
+## --no-only
+This is particularly useful in a CI environment where an [only test](#testonlyname-opts-cb) is not supposed to go unnoticed.
+
+By passing the `--no-only` flag, any existing [only test](#testonlyname-opts-cb) causes tests to fail.
+
+```sh
+tape **/*.js --no-only
+```
+
+Alternatively, environmental variable `NODE_TAPE_NO_ONLY_TEST` can be set to `true` to achieve same behavior. Do note that the command-line flag takes precedence.
+
 # methods
 
 The assertion methods in `tape` are heavily influenced or copied from the methods in [node-tap](https://github.com/isaacs/node-tap).