add mini readme for how to consume/add proposals, #131165

src/vscode-dts/README.md

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+
+## vscode-dts
+
+This is the place for the stable API and for API proposals.
+
+
+### Consume a proposal
+
+1. find a proposal you are interested in
+1. add its name to your extensions `package.json#enabledApiProposals` property
+1. run `npx vscode-dts dev` to download the `d.ts` files into your project
+1. don't forget that extension using proposed API cannot be published
+1. learn more here: https://code.visualstudio.com/api/advanced-topics/using-proposed-api
+
+### Add a new proposal
+
+1. create a _new_ file in this directory, its name must follow this pattern `vscode.proposed.[a-zA-Z]+.d.ts`
+1. creating the proposal-file will automatically update `src/vs/workbench/services/extensions/common/extensionsApiProposals.ts` (make to run `yarn watch`)
+1. declare and implement your proposal
+1. make sure to use the `checkProposedApiEnabled` and/or `isProposedApiEnabled`-utils to enforce the API being proposed. Make sure to invoke them with your proposal's name which got generated into `extensionsApiProposals.ts`