Add section regarding Pkg mode to REPL docs (#38842)

* Add section regarding Pkg mode to REPL docs

Co-authored-by: Stephen Vavasis <vavasis@uwaterloo.ca>
(cherry picked from commit ce338db)

stdlib/REPL/docs/src/index.md

@@ -22,7 +22,7 @@ and a `julia>` prompt.
 
 ### The Julian mode
 
-The REPL has four main modes of operation. The first and most common is the Julian prompt. It
+The REPL has five main modes of operation. The first and most common is the Julian prompt. It
 is the default mode of operation; each new line initially starts with `julia>`. It is here that
 you can enter Julia expressions. Hitting return or enter after a complete expression has been
 entered will evaluate the entry and show the result of the last expression.
@@ -188,6 +188,14 @@ C:\Users\elm>dir
 02/02/2020  08:06    <DIR>          .atom
 ```
 
+### Pkg mode
+
+The Package manager mode accepts specialized commands for loading and updating packages. It is entered
+by pressing the `]` key at the Julian REPL prompt and exited by pressing CTRL-C or pressing the backspace key
+at the beginning of the line. The prompt for this mode is `pkg>`. It supports its own help-mode, which is
+entered by pressing `?` at the beginning  of the line of the `pkg>` prompt. The Package manager mode is
+documented in the Pkg manual, available at [https://julialang.github.io/Pkg.jl/v1/](https://julialang.github.io/Pkg.jl/v1/).
+
 ### Search modes
 
 In all of the above modes, the executed lines get saved to a history file, which can be searched.