[web] Document the need for yarn classic (#745)

People often run into issues with `yarn install` because they're using a
newer yarn. The situation is generally bad - we don't want to update to
Yarn v4 yet because it is marked experimental and is not the default
yarn that gets installed by node currently. We could add a
`packageManager` field to our package.json, but this will only fail the
build with a better (hopefully) error message, and will necessitate the
user to `corepack_enable`.

I'm not sure what's the best approach right now to make the initial
setup be seamless (I think we're using the approach that works for the
maximum of all the alternatives, but I'm not sure). At least, let me add
a note about it.

Ref:
* yarnpkg/berry#5912

web/README.md

@@ -32,8 +32,8 @@ yarn dev
 
 That's it. The web app will automatically hot reload when you make changes.
 
-If you're new to web development and unsure about how to get started, see
-[docs/new](docs/new.md).
+If you're new to web development and unsure about how to get started,  or are
+facing some problems when running the above steps, see [docs/new](docs/new.md).
 
 ## Other apps
 

web/docs/new.md

@@ -13,3 +13,14 @@ development, here is a recommended workflow:
    `yarn` comes with it.
 
 That's it. Enjoy coding!
+
+### Yarn
+
+Note that we use Yarn classic
+
+```
+$ yarn --version
+1.22.21
+```
+
+You should be seeing a 1.xx.xx version, otherwise your `yarn install` will fail.