fix: Clearly label version compatibility

closes #338

Great idea from @ThatBrianDude

README.md

@@ -18,7 +18,7 @@
 
 ## Features
 - Extremely easy deployment
-- The widest OpenAPI features support (yes, it supports even `discriminator`) <br>
+- The widest OpenAPI v2.0 features support (yes, it supports even `discriminator`) <br>
 ![](docs/images/discriminator-demo.gif)
 - Neat **interactive** documentation for nested objects <br>
 ![](docs/images/nested-demo.gif)
@@ -32,6 +32,7 @@
 - Multiple ReDoc instances on single page ([example](demo/examples/multiple-apis/index.html))
 
 ## Roadmap
+  - [ ] [OpenAPI v3.0 support](https://github.com/Rebilly/ReDoc/issues/312)
   - [x] ~~performance optimizations~~
   - [x] ~~better navigation (menu improvements + search)~~
   - [ ] ability to simple branding/styling
@@ -44,6 +45,12 @@ We host the latest and all the previous ReDoc releases on GitHub Pages-based **C
 - `v1.x.x` release: https://rebilly.github.io/ReDoc/releases/v1.x.x/redoc.min.js
 - `latest` release: https://rebilly.github.io/ReDoc/releases/latest/redoc.min.js this file is updated with each release of ReDoc and may introduce breaking changes. **Not recommended to use in production.** Use particular release or `v1.x.x`.
 
+## Version Guidance
+| ReDoc Release | OpenAPI Specification |
+|:--------------|:----------------------|
+| 1.18.x        | 2.0                   |
+| 1.17.x        | 2.0                   |
+
 ## Some Real-life usages
 - [Rebilly](https://rebilly.github.io/RebillyAPI)
 - [Docker Engine](https://docs.docker.com/engine/api/v1.25/)