docs: add 3 vendor extensions to list (#1457)

README.md

@@ -219,6 +219,9 @@ ReDoc makes use of the following [vendor extensions](https://swagger.io/specific
 * [`x-servers`](docs/redoc-vendor-extensions.md#x-servers) - ability to specify different servers for API (backported from OpenAPI 3.0)
 * [`x-ignoredHeaderParameters`](docs/redoc-vendor-extensions.md#x-ignoredHeaderParameters) - ability to specify header parameter names to ignore
 * [`x-additionalPropertiesName`](docs/redoc-vendor-extensions.md#x-additionalPropertiesName) - ability to supply a descriptive name for the additional property keys
+* [`x-summary`](docs/redoc-vendor-extensions.md#x-summary) - For Response object, use as the response button text, with description rendered under the button
+* [`x-extendedDiscriminator`](docs/redoc-vendor-extensions.md#x-extendedDiscriminator) - In Schemas, uses this to solve name-clash issues with the standard discriminator
+* [`x-explicitMappingOnly`](docs/redoc-vendor-extensions.md#x-explicitMappingOnly) - In Schemas, display a more descriptive property name in objects with additionalProperties when viewing the property list with an object
 
 ### `<redoc>` options object
 You can use all of the following options with standalone version on <redoc> tag by kebab-casing them, e.g. `scrollYOffset` becomes `scroll-y-offset` and `expandResponses` becomes `expand-responses`.