update doc

Author: ko1

README.md

@@ -499,6 +499,7 @@ config set no_color true
   * `RUBY_DEBUG_SOCK_PATH` (`sock_path`): UNIX Domain Socket remote debugging: socket path
   * `RUBY_DEBUG_SOCK_DIR` (`sock_dir`): UNIX Domain Socket remote debugging: socket directory
   * `RUBY_DEBUG_LOCAL_FS_MAP` (`local_fs_map`): Specify local fs map
+  * `RUBY_DEBUG_IGNORE_BP` (`ignore_bp`): Ignore breakpoints if no clients are attached (default: false)
   * `RUBY_DEBUG_COOKIE` (`cookie`): Cookie for negotiation
   * `RUBY_DEBUG_OPEN_FRONTEND` (`open_frontend`): frontend used by open command (vscode, chrome, default: rdbg).
   * `RUBY_DEBUG_CHROME_PATH` (`chrome_path`): Platform dependent path of Chrome (For more information, See [here](https://github.com/ruby/debug/pull/334/files#diff-5fc3d0a901379a95bc111b86cf0090b03f857edfd0b99a0c1537e26735698453R55-R64))