docs: add installation docs

docs/installation.md

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+# Installation
+
+`HttpRunner+` is developed with Golang, it supports Go `1.13+` and most operating systems. Combination of Go `1.13/1.14/1.15/1.16/1.17` and `macOS/Linux/Windows` are tested continuously on [GitHub-Actions][github-actions].
+
+## install as CLI tool
+
+```bash
+$ go get -u github.com/httprunner/hrp/hrp
+```
+
+Since installed, you will get a `hrp` command with multiple sub-commands.
+
+```text
+$ hrp -h
+hrp (HttpRunner+) is the next generation for HttpRunner. Enjoy! ✨ 🚀 ✨
+
+License: Apache-2.0
+Github: https://github.com/httprunner/hrp
+Copyright 2021 debugtalk
+
+Usage:
+  hrp [command]
+
+Available Commands:
+  boom        run load test with boomer
+  completion  generate the autocompletion script for the specified shell
+  har2case    Convert HAR to json/yaml testcase files
+  help        Help about any command
+  run         run API test
+
+Flags:
+  -h, --help               help for hrp
+      --log-json           set log to json format
+  -l, --log-level string   set log level (default "INFO")
+  -v, --version            version for hrp
+
+Use "hrp [command] --help" for more information about a command.
+```
+
+## install as library
+
+Beside using `hrp` as a CLI tool, you can also use it as golang library.
+
+```bash
+$ go get -u github.com/httprunner/hrp
+```
+
+Then you can import `github.com/httprunner/hrp` and write testcases in Golang.
+
+[github-actions]: https://github.com/httprunner/hrp/actions