doc/syntax: add doc for string interpolation
@@ -81,6 +81,8 @@ Strings are enclosed by `" ... "` for a single line string or by `m#" ... "#m` f
They can be concatenated with the operator `++`.
Strings must be UTF-8 valid.
+String interpolation exists in Nickel, via the notation `"#{ < expression that evaluates to a string > }"`.
+
Examples:
```
> "Hello, World!"
@@ -93,6 +95,15 @@ Yes it is, indeed it is"
> "Hello" ++ "World"
"HelloWorld"
+
+> let h = "Hello" in "#{h} World"
+"Hello World"
+
+> let n = 5 in "The number #{n}."
+error: Type error
+
+> let n = 5 in "The number #{strings.from_num n}."
+"The number 5."
```
## Equality Operators
