Added Recommended Design Elements & Resources Section to Lovelace Custom Card Docs (#313)

* Added Recommended Design Elements & Resources

As a new dev to HA & to custom-card building, this would have been very helpful.
- Recommended Design Elements help us all to stay in sync on using the same UI/UX Designs.
- I found the boilerplate card VERY helpful when trying to get started.

* Update lovelace_custom_card.md

* Updated Adding a Second Resource

* Updated Lovelace Custom Card Docs

Renamed 'Resources to Advanced Resources'
Tweaked description on Boilerplate Card

docs/lovelace_custom_card.md

@@ -223,3 +223,19 @@ views:
       - input_boolean.switch_ac_livingroom
       - input_boolean.switch_tv
 ```
+
+## Recommended Design Elements
+We are currently migrating from using Paper Elements to MWC (Material Web Component) Elements.
+
+If an element exists in the below repository for MWC. We recommended using it.
+- [MWC (Material Web Components)](https://material-components.github.io/material-components-web-components/demos/index.html)
+
+If an element does not exist in MWC, we default to using Paper Elements.
+- [Paper Elements](https://www.webcomponents.org/collection/PolymerElements/paper-elements)
+
+## Advanced Resources
+Community Maintained Boiler Plate Card - Advanced Template (Typescript, Rollup, Linting, etc.)
+- [Boilerplate Card](https://github.com/custom-cards/boilerplate-card)
+
+Developer Documentation for [HACS](https://hacs.netlify.com/) (Home Assitant Community Store).
+- [HACS Plugin Docs](https://hacs.netlify.com/developer/plugin/)