Extend README (#33)

Add badges and instructions.

README.md

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 # selectel-mks-cli: CLI for the Selectel Managed Kubernetes Service
 
+[![crates.io](https://img.shields.io/crates/v/selectel-mks-cli.svg)](https://crates.io/crates/selectel-mks-cli)
+[![Documentation](https://docs.rs/selectel-mks-cli/badge.svg)](https://docs.rs/selectel-mks-cli)
+![CI](https://github.com/ozerovandrei/selectel-mks-cli/workflows/CI/badge.svg?branch=master)
+
 CLI to the Selectel MKS V1 API.
 
+## Installation
+
+Download a binary for the needed platform from the releases page.
+
+## Usage
+
+You need to specify two mandatory parameters to use CLI:
+
+ * `MKS endpoint` that can be specified by the `--mks-endpoint` option or `MKS_ENDPOINT` environment variable;
+ * `MKS project-scoped token` that can be specified by the `--mks-token` option or `MKS_TOKEN` environment variable.
+
+You can use `help` command to see all available subcommands:
+
+```bash
+$ mks help
+mks 0.1.0
+
+USAGE:
+    mks [FLAGS] --mks-endpoint <mks-endpoint> --mks-token <mks-token> <SUBCOMMAND>
+
+FLAGS:
+    -d, --debug      Activate debug mode
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+    -v, --verbose    Activate verbose mode
+
+OPTIONS:
+        --mks-endpoint <mks-endpoint>    MKS endpoint [env: MKS_ENDPOINT]
+        --mks-token <mks-token>          MKS project-scoped token [env: MKS_TOKEN]
+
+SUBCOMMANDS:
+    cluster        Cluster commands
+    help           Prints this message or the help of the given subcommand(s)
+    kubeversion    Kubeversion commands
+    node           Node commands
+    nodegroup      Nodegroup commands
+    task           Task commands
+```
+
+You can also use `help` with any subcommand to see all available nested subcommands and their options:
+
+```bash
+$ mks help nodegroup
+mks-nodegroup 0.1.0
+Nodegroup commands
+
+USAGE:
+    mks nodegroup <SUBCOMMAND>
+
+FLAGS:
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+SUBCOMMANDS:
+    create    Create a new nodegroup
+    delete    Delete nodegroup
+    get       Get cluster nodegroup
+    help      Prints this message or the help of the given subcommand(s)
+    list      List cluster nodegroups
+    set       Set nodegroup parameters
+``` 
+
+```bash
+$ mks help nodegroup set
+mks-nodegroup-set 0.1.0
+Set nodegroup parameters
+
+USAGE:
+    mks nodegroup set [OPTIONS] <nodegroup-id> --cluster-id <cluster-id>
+
+FLAGS:
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+OPTIONS:
+        --cluster-id <cluster-id>      Cluster identifier
+        --nodes-count <nodes-count>    Count of nodes
+
+ARGS:
+    <nodegroup-id>    Nodegroup identifier
+```
+
+## How to get available values for mks-endpoint option
+
+There are separate endpoints for each MKS region:
+
+```bash
+ru-1: https://ru-1.mks.selcloud.ru/v1
+ru-2: https://ru-2.mks.selcloud.ru/v1
+ru-3: https://ru-3.mks.selcloud.ru/v1
+ru-7: https://ru-7.mks.selcloud.ru/v1
+ru-8: https://ru-8.mks.selcloud.ru/v1
+```
+
+You can get available endpoints from the endpoint catalog in the Selectel Cloud.
+
+## How to get mks-token value
+
+You can see this token in your web-browser console when working with the Selectel Cloud [API](https://developers.selectel.ru/docs/selectel-cloud-platform/main-services/selectel_cloud_management_api/).  
+You can also create this token with [Terraform](https://registry.terraform.io/providers/selectel/selectel/latest/docs/resources/vpc_token_v2), [Go library](https://pkg.go.dev/github.com/selectel/[email protected]/selvpcclient/resell/v2/tokens?tab=doc) or [Python library/CLI](https://github.com/selectel/python-selvpcclient).
+
 ## License
 
 Licensed under either of