Add urfave_cli_no_docs build tag

This removes the resulting binary dependency on cpuguy83/md2man and
russross/blackfriday, which saves more than 400 KB (more than 300 KB
once stripped) from the resulting binary.

Document this in README.

(cherry picked from commit 49e43be)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

README.md

@@ -39,6 +39,16 @@ cli is tested against multiple versions of Go on Linux, and against the latest
 released version of Go on OS X and Windows.  For full details, see
 [`./.travis.yml`](./.travis.yml) and [`./appveyor.yml`](./appveyor.yml).
 
+### 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 less dependencies.
+
 ### Using `v1` releases
 
 ```

docs.go

@@ -1,3 +1,6 @@
+//go:build !urfave_cli_no_docs
+// +build !urfave_cli_no_docs
+
 package cli
 
 import (

docs_test.go

@@ -1,3 +1,6 @@
+//go:build !urfave_cli_no_docs
+// +build !urfave_cli_no_docs
+
 package cli
 
 import (