Add discord invite to contributing doc (paritytech#14674)

* Rename file

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Convert to Markdown

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Add discord badge

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

* Fix links

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

---------

Signed-off-by: Oliver Tale-Yazdi <[email protected]>

README.md

@@ -1,4 +1,4 @@
-# Substrate · [![GitHub license](https://img.shields.io/badge/license-GPL3%2FApache2-blue)](#LICENSE) [![GitLab Status](https://gitlab.parity.io/parity/substrate/badges/master/pipeline.svg)](https://gitlab.parity.io/parity/substrate/pipelines) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/CONTRIBUTING.adoc) [![Stack Exchange](https://img.shields.io/badge/Substrate-Community%20&%20Support-24CC85?logo=stackexchange)](https://substrate.stackexchange.com/)
+# Substrate · [![GitHub license](https://img.shields.io/badge/license-GPL3%2FApache2-blue)](#LICENSE) [![GitLab Status](https://gitlab.parity.io/parity/substrate/badges/master/pipeline.svg)](https://gitlab.parity.io/parity/substrate/pipelines) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/CONTRIBUTING.md) [![Stack Exchange](https://img.shields.io/badge/Substrate-Community%20&%20Support-24CC85?logo=stackexchange)](https://substrate.stackexchange.com/)
 <p align="center">
   <img src="/docs/media/sub.gif">
 </p>
@@ -18,7 +18,7 @@ Please do report bugs and [issues here](https://github.com/paritytech/substrate/
 
 ## Contributions & Code of Conduct
 
-Please follow the contributions guidelines as outlined in [`docs/CONTRIBUTING.adoc`](docs/CONTRIBUTING.adoc).
+Please follow the contributions guidelines as outlined in [`docs/CONTRIBUTING.md`](docs/CONTRIBUTING.md).
 In all communications and contributions, this project follows the [Contributor Covenant Code of Conduct](docs/CODE_OF_CONDUCT.md).
 
 ## Security
@@ -33,4 +33,3 @@ The security policy and procedures can be found in [`docs/SECURITY.md`](docs/SEC
 The reason for the split-licensing is to ensure that for the vast majority of teams using Substrate to create feature-chains, then all changes can be made entirely in Apache2-licensed code, allowing teams full freedom over what and how they release and giving licensing clarity to commercial teams.
 
 In the interests of the community, we require any deeper improvements made to Substrate's core logic (e.g. Substrate's internal consensus, crypto or database code) to be contributed back so everyone can benefit.
-