Un-delete non-redundant docs

docs/index.md

@@ -19,3 +19,55 @@ In addition to the version-specific guides, these other documents are available:
 - [CONTRIBUTING](./CONTRIBUTING/)
 - [CODE OF CONDUCT](./CODE_OF_CONDUCT/)
 - [RELEASING](./RELEASING/)
+
+## Installation
+
+Using this package requires a working Go environment. [See the install instructions for Go](http://golang.org/doc/install.html).
+
+Go Modules are required when using this package. [See the go blog guide on using Go Modules](https://blog.golang.org/using-go-modules).
+
+### Using `v2` releases
+
+```
+$ go get github.com/urfave/cli/v2
+```
+
+```go
+...
+import (
+  "github.com/urfave/cli/v2" // imports as package "cli"
+)
+...
+```
+
+### Using `v1` releases
+
+```
+$ go get github.com/urfave/cli
+```
+
+```go
+...
+import (
+  "github.com/urfave/cli"
+)
+...
+```
+
+### Build tags
+
+You can use the following build tags:
+
+#### `urfave_cli_no_docs`
+
+When set, this removes `ToMarkdown` and `ToMan` methods, so your application
+won't be able to call those. This reduces the resulting binary size by about
+300-400 KB (measured using Go 1.18.1 on Linux/amd64), due to fewer dependencies.
+
+### Supported platforms
+
+cli is tested against multiple versions of Go on Linux, and against the latest
+released version of Go on OS X and Windows. This project uses Github Actions
+for builds. To see our currently supported go versions and platforms, look at
+the [github workflow
+configuration](https://github.com/urfave/cli/blob/main/.github/workflows/cli.yml).