readonly: add page (#10786)

* readonly: add page

* Update readonly.md

* Update readonly.md

* Update pages/common/readonly.md

Co-authored-by: Reinhart Previano Koentjoro <reinhart_previano@yahoo.com>

* Update readonly.md

* Update pages/common/readonly.md

Co-authored-by: Jack Lin <blueskyson1401@gmail.com>

* Update pages/common/readonly.md

* Update pages/common/readonly.md

* Update pages/common/readonly.md

---------

Co-authored-by: Reinhart Previano Koentjoro <reinhart_previano@yahoo.com>
Co-authored-by: Jack Lin <blueskyson1401@gmail.com>

pages/common/readonly.md

@@ -0,0 +1,17 @@
+# readonly
+
+> Create or modify read-only variables within a shell script, preventing the variable from being changed by subsequent commands.
+> This is useful when you want to ensure that a variable retains a constant value throughout the execution of a script.
+> More information: <https://manned.org/readonly.1p>.
+
+- Create a read-only variable:
+
+`readonly {{variable_name}}={{value}}`
+
+- Mark a variable as read-only:
+
+`readonly {{existing_variable}}`
+
+- [p]rint to the standard output the names and values of all read-only variables:
+
+`readonly -p`