feat: add script to build platform specific standalone binaries

This revision adds a dependency on the 'pkg' tool to create standalone binaries
for Linux, MacOS, and Windows that do not depend on node.js being installed. To
run the tool, run 'npm run pkg' and the binaries will be created in a 'bin'
directory in the root.

* Changed pkg command to use local cli version of pkg to eliminate requirement that it be installed globally.

.gitignore

@@ -6,3 +6,4 @@ dist
 .validaterc
 .validateignore
 .idea/
+bin/

README.md

@@ -50,6 +50,11 @@ The `-g` flag installs the tool globally so that the validator can be run from a
 3. Install the dependencies using `npm install`
 4. Build the command line tool by running `npm run link`.
 
+### Platform specific binaries
+It is possible to build platform specific binaries for Linux, MacOS, and Windows that do not depend on having node.js installed.
+
+To build these, run `npm run pkg` in the root.  The binaries (lint-openapi-linux, lint-openapi-macos, lint-openapi-windows.exe respectively) are built in the 'bin' directory in the root.
+
 ## Usage
 ### Command line
 `lint-openapi [options] [command] [<files>]`