[readme] remove avn from readme

`avn` is archived ([see repo](https://github.com/direnv/direnv)) and should probably no longer be recommended by nvm.

This also adds a solution using `direnv` and proposes a possible `.envrc` to use.
See [wbyoung/avn#109](wbyoung/avn#109 (comment)) for more suggested `.envrc` files.

Author: andrew-chang-dewitt

README.md

@@ -588,9 +588,7 @@ Run [`npx nvmrc`](https://npmjs.com/nvmrc) to validate an `.nvmrc` file. If that
 
 ### Deeper Shell Integration
 
-You can use [`avn`](https://github.com/wbyoung/avn) to deeply integrate into your shell and automatically invoke `nvm` when changing directories. `avn` is **not** supported by the `nvm` maintainers. Please [report issues to the `avn` team](https://github.com/wbyoung/avn/issues/new).
-
-You can also use [`nvshim`](https://github.com/iamogbz/nvshim) to shim the `node`, `npm`, and `npx` bins to automatically use the `nvm` config in the current directory. `nvshim` is **not** supported by the `nvm` maintainers. Please [report issues to the `nvshim` team](https://github.com/iamogbz/nvshim/issues/new).
+You can use [`nvshim`](https://github.com/iamogbz/nvshim) to shim the `node`, `npm`, and `npx` bins to automatically use the `nvm` config in the current directory. `nvshim` is **not** supported by the `nvm` maintainers. Please [report issues to the `nvshim` team](https://github.com/iamogbz/nvshim/issues/new).
 
 If you prefer a lighter-weight solution, the recipes below have been contributed by `nvm` users. They are **not** supported by the `nvm` maintainers. We are, however, accepting pull requests for more examples.