docs: Add Authgear to Readme file

[Boburmirzo]

Motivation

I thought it can be helpful for the community here to use such tools.

Authgear has many free-to-use SDKs for different platforms. You can check the repos here:

https://github.com/authgear

About Authgear shortly:

Authgear acts as an IAM provider that is a gatekeeper to the resources and the gatekeeper initiates authorization as outlined in OAuth 2.0. The addition of the OpenID Connect layer adds authentication to secure users’ digital identities.

Affiliation

I am contributing to open-source authentication SDKs there.

• [x] I am the author of the article or project
• [ ] I am working for/with the company which is publishing the article or project
• [ ] I'm just a rando who stumbled upon this via social networks

Self checks

• [x] I have read the Code of Conduct
• [x] I applied all rules from the Contributing guide
• [x] I have checked there is no other Issues or Pull Requests covering the same topic to open

[Boburmirzo]

@kdeldycke Could you check this PR, please?