feat: adds string literal replacement syntax

Signed-off-by: Vincent Biret <vincentbiret@hotmail.com>

Author: baywet

versions/1.2.0-dev.md

@@ -157,6 +157,33 @@ The properties of the `update` or `copy` object MUST be compatible with the targ
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).
 
+### String literal replacement syntax
+
+The following string replacement syntax MAY be used:
+
+%source.key%
+
+Where **source** is a known source which value MUST ONLY be **env** for environment variables, or **param** for parameters. The key MUST only contain the following characters A-Za-z0-9, contain at least one character and start with a letter.
+
+#### ABNF notation
+
+```abnf
+replacement-string = "%" source "." key "%"
+source             = "env" / "param"
+key                = ALPHA *( ALPHA / DIGIT )
+```
+
+Where ALPHA and digit rules are defined in [[RFC5234]].
+
+#### Environment replacement source
+
+When the environment replacement source is used, the key MUST match (case-insensitive) an environment variable defined for the process parsing the Overlay document. If the environment variable is not defined, the processor MAY either replace it with an empty string or return an error.
+
+#### Parameter replacement source
+
+When the parameter replacement source is used, the key MUST match (case-sensitive) a corresponding parameter defined for the action template.
+
+
 ### Examples
 
 #### Structured Overlay Example