tools/mmgotool: move into monorepo (#24010)

tools/README.md

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+# Tools
+
+This directory aims to provide a set of tools that simplify and enhance various development tasks. This README file serves as a guide to help you understand the directory, features of these tools, and how to get started using it. This is a collection of utilities and scripts designed to streamline common development tasks for Mattermost. These tools aim to help automate repetitive tasks and improve productivity.
+
+## Included tools
+
+* **mmgotool**: is a CLI to help with i18n related checks for the mattermost/server development.
+
+## Installation & Usage
+
+### mmgotool
+
+To install `mmgotool`, simply run the following command: `go install github.com/mattermost/mattermost/tools/mmgotool`
+
+Make sure you have the necessary prerequisites such as [Go](https://go.dev/) compiler.
+
+`mmgotool i18n` has following subcommands described below:
+
+* `check`: Check translations
+* `check-empty-src`: Check for empty translation source strings
+* `clean-empty`: Clean empty translations
+* `extract`: Extract translations