Added OpenWrt Wireguard client manual

[haltunbay]

Description

This change includes manual for configuring wireguard vpn client for a router with openwrt.

Motivation and Context

This scenario is useful in case you want to use vpn with devices which has no vpn capability like smart tv, or make vpn connection available via router for multiple devices.
Solves following issue:
#624

How Has This Been Tested?

This is a tested, working scenario with following environment:

• algo installed ubuntu at digitalocean
• client side router "TP-Link TL-WR1043ND" with openwrt ver. 21.02.1. See compatible device list at https://openwrt.org/toh/start . Theoretically any of the device on list should work

Types of changes

• Documentation update

Checklist:

• [] I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [] I have added tests to cover my changes.
• [] All new and existing tests passed.

[CLAassistant]

All committers have signed the CLA.