Add import plugin to give it docs (#2428)

* wip: add import to make it have docs

Fake the import plugin further to give it proper docs. This should help
make it more discoverable.
Signed-off-by: Miek Gieben <miek@miek.nl>

* Finish writing the documentation
Signed-off-by: Miek Gieben <miek@miek.nl>

corefile.5.md

@@ -39,6 +39,8 @@ Comments may be started anywhere on a line.
 Environment variables are supported and either the Unix or Windows form may be used: `{$ENV_VAR_1}`
 or `{%ENV_VAR_2%}`.
 
+You can use the `import` "plugin" to include parts of other files.
+
 If CoreDNS can’t find a Corefile to load it loads the following builtin one:
 
 ~~~ corefile
@@ -49,8 +51,7 @@ If CoreDNS can’t find a Corefile to load it loads the following builtin one:
 
 ## Import
 
-You can use the `import` "plugin" to include parts of other files, see <https://coredns.io/explugins/import>.
-This is enabled by default.
+You can use the `import` "plugin" to include parts of other files, see <https://coredns.io/plugins/import>.
 
 ## Snippets
 
@@ -133,3 +134,4 @@ Apache License 2.0
 ## See Also
 
 The manual page for CoreDNS: coredns(1) and more documentation on <https://coredns.io>.
+Also see the [*import*](https://coredns.io/plugins/import)'s documentation.

plugin/import/import.md

+# import
+
+## Name
+
+*import* - include files or reference snippets from a Corefile.
+
+## Description
+
+The *import* plugin can be used to incude files into the main configuration. Another use it to
+reference predefined snippets. Both can help to avoid some duplication.
+
+This is a unique directive in that *import* can appear outside of a server block. In other words, it
+can appear at the top of a Corefile where an address would normally be.
+
+## Syntax
+
+~~~
+import PATTERN
+~~~
+
+*   **PATTERN** is the file, glob pattern (`*`) or snippet to include. Its contents will replace
+    this line, as if that file's contents appeared here to begin with.
+
+## Files
+
+You can use *import* to include a file or files. This file's location is relative to the
+Corefile's location. It is an error if a specific file cannot be found, but an empty glob pattern is
+not an error.
+
+## Snippets
+
+You can define snippets to be reused later in your Corefile by defining a block with a single-token
+label surrounded by parentheses:
+
+~~~ corefile
+(mysnippet) {
+	...
+}
+~~~
+
+Then you can invoke the snippet with *import*:
+
+~~~
+import mysnippet
+~~~
+
+## Examples
+
+Import a shared configuration:
+
+~~~
+. {
+   import config/common.conf
+}
+~~~
+
+Where `config/common.conf` contains:
+
+~~~
+prometheus
+errors
+log
+~~~
+
+This imports files found in the zones directory:
+
+~~~
+import ../zones/*
+~~~
+
+## Also See
+
+See corefile(5).