docs: add npm link documentation for local development (apache#201)

* NPM link docs (hat tip to Krist)

* formatting changes in prior commit

superset-frontend/temporary_superset_ui/superset-ui/plugins/superset-ui-plugins/README.md

@@ -86,6 +86,8 @@ superset-ui/
     ...
 ```
 
+
+
 ### Installation
 
 1. clone this repo
@@ -97,6 +99,69 @@ yarn install
 yarn build
 ```
 
+### Activating plugins for local development
+
+1. Enable `npm link` for the package.
+
+```sh
+cd superset-ui
+cd packages/superset-ui-chart
+npm link
+```
+
+2. Link the local package to `incubator-superset`.
+
+```sh
+cd incubator-superset
+cd superset/assets
+npm link @superset-ui/chart \# use package name in package.json, not directory name 
+```
+
+3) After npm link complete, update the import statements in Superset.
+
+Instead of
+
+```js
+import { xxx } from '@superset-ui/plugin-chart-horizon';
+```
+
+which will point to the transpiled code.
+
+do refer to `src`
+
+```js
+import { xxx } from '@superset-ui/plugin-chart-horizon/src'
+```
+
+4. After that you can run `dev-server` as usual.
+
+```sh
+npm run dev-server
+```
+
+Now when you change the code in `@superset-ui`, it will update the app immediately similar to code inside `incubator-superset`.
+
+### Deactivating plugins for local development
+
+1. Change the `import` statements back.
+
+2. Unlink the package from `incubator-superset`.
+
+```cd incubator-superset
+cd superset/assets
+npm unlink @superset-ui/chart
+```
+
+Note: Quite often, `npm link` mess up your `node_modules` and the `unlink` command above does not work correctly, making webpack build fails or other unexpected behaviors. If that happens, just delete your `node_modules` and `npm install` from scratch.
+
+3. Clean up global link.
+
+```sh
+cd superset-ui
+cd packages/superset-ui-chart
+npm unlink
+```
+
 ### Builds, linting, and testing
 
 Each package defines its own build config, linting, and testing. You can have lerna run commands