feat: add license file

[wayne-shih]

Description

Please include a summary of changes.

Noticed the README file states the repo is licensed under SIL OFL, there's LICENSE files in the Sans and Serif folders none in the master dir for github to easily recognise the license. This PR is to add one in. After –

Wording used on the master SIL license file:

Copyright 2014-2017 Indian Type Foundry (info@indiantypefoundry.com). 
Copyright 2014-2018 Adobe (http://www.adobe.com/), with Reserved Font Name 'Source'. All Rights Reserved. Source is a trademark of Adobe in the United States and/or other countries. 
Copyright 2019 Google LLC.

This Font Software is licensed under the SIL Open Font License, Version 1.1.
...

ITF copyright attribution is for DM Sans; Adobe copyright attribution is for DM Serif; Google is for both as per stand alone SIL license copyright attribution statements.

Does this code break existing functionality?

No

[m4rc1e]

license should be named OFL.txt. Please take the OFL.txt from https://github.com/google/fonts/tree/master/ofl/dmmono

[wayne-shih]

license should be named OFL.txt. Please take the OFL.txt from https://github.com/google/fonts/tree/master/ofl/dmmono

I've amended the license file name to OFL.txt This repo's for DM Sans and DM Serif, so have replaced it with relevant OFL file from the @google/fonts repo.

• https://github.com/google/fonts/tree/master/ofl/dmsans/OFL.txt
• https://github.com/google/fonts/tree/master/ofl/dmserifdisplay/OFL.txt
• https://github.com/google/fonts/tree/master/ofl/dmseriftext/OFL.txt

Hope this is fine now? thanks :)

[m4rc1e]

Apologies. I thought I was looking at the monospaced repo.

It seems each font dir (Serif, Sans etc) already have a LICENSE.txt file which contain the correct OFL.txt text.

[m4rc1e]

Not opposed to merging this since you've consolidated all holders into the parent dir. Thanks