Add documentation for --save-bundle flag (#386)

Author: just-boris

packages/size-limit/create-help.js

@@ -42,6 +42,7 @@ export default process => {
       `  ${y('--watch')}           Runs in watch mode`,
       `  ${y('--silent')}          Show only failed limits`,
       `  ${y('--debug')}           Show internal configs for issue report`,
+      `  ${y('--save-bundle DIR')} Write output bundle to disk for manual review`,
       `  ${y('--version')}         Display version`
     )
     if (plugins.has('webpack')) {

packages/size-limit/test/__snapshots__/run.test.js.snap

@@ -223,6 +223,7 @@ Check the real performance cost of your front-end project to users
   --watch           Runs in watch mode
   --silent          Show only failed limits
   --debug           Show internal configs for issue report
+  --save-bundle DIR Write output bundle to disk for manual review
   --version         Display version
 
 Examples:
@@ -323,6 +324,7 @@ Check the real performance cost of your front-end project to users
   --watch           Runs in watch mode
   --silent          Show only failed limits
   --debug           Show internal configs for issue report
+  --save-bundle DIR Write output bundle to disk for manual review
   --version         Display version
 
 Webpack options:
@@ -554,6 +556,7 @@ Check the real performance cost of your front-end project to users
   --watch           Runs in watch mode
   --silent          Show only failed limits
   --debug           Show internal configs for issue report
+  --save-bundle DIR Write output bundle to disk for manual review
   --version         Display version
 
 Webpack options: