On hacklang.org, locate the 'Readonly' section within the documentation table of contents. Based on the brief description provided, explain what 'Readonly' is primarily designed for in Hack.
Navigate to hacklang.org. Scroll down to locate the 'Control Flow' section in the table of contents. Identify the 'Readonly' entry under 'Control Flow'. Extract the brief description associated with 'Readonly'.
Successfully identifies the 'Readonly' entry in the table of contents. States that 'Readonly' is a specialized feature for performantly controlling object mutability.