Update cheatsheets

readonly

@@ -0,0 +1,21 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# readonly
+
+> Set read-only shell variables.
+> More information: <https://manned.org/readonly.1posix>.
+
+- Set a read-only variable:
+
+`readonly {{variable_name}}={{value}}`
+
+- Mark a variable as read-only:
+
+`readonly {{existing_variable}}`
+
+- [p]rint the names and values of all read-only variables to `stdout`:
+
+`readonly -p`