doc: Add allowlist pattern for .nowignore (#1615)

* doc: Add whitelisting approach

Inspiration taken from this discussion:
https://spectrum.chat/zeit/general/whitelisting-with-nowignore-not-straightforward-needs-to-be-in-docs~9e260aec-3d7c-49a2-ac38-a31d2ee92dce?m=MTU0ODMzODY0ODQxNQ==

* Apply suggestions from code review

Co-Authored-By: Steven <[email protected]>

Co-authored-by: Steven <[email protected]>
Co-authored-by: Matthew Sweeney <[email protected]>

Author: 3 people

pages/guides/prevent-uploading-sourcepaths-with-nowignore.mdx

@@ -32,6 +32,23 @@ private.html
   An example <InlineCode>.nowignore</InlineCode> file.
 </Caption>
 
+## Allowlist
+
+A typical `.nowignore` file assumes all files are allowed and each entry is a pattern to ignore. Alternatively, you may chose to ignore all files and each entry is a pattern to allow.
+The first step is to add a wildcard `/*` as the first line in `.nowignore`. This will ensure all directories and files in the project root are ignored. The following lines must start with a `!` to invert the ignore action and ensure the directory or file is allowed.
+
+```
+# Ignore everything (folders and files) on root only
+/*
+!api
+!now.json
+!*.html
+```
+
+<Caption>
+  An example <InlineCode>.nowignore</InlineCode> file using the "allowlist" pattern.
+</Caption>
+
 ## Uploaded Files
 
 Aside from the [default exclusions](/docs/v2/build-step/#ignored-files-and-folders), all files within your project are uploaded to ZEIT Now if no source path is specified to be excluded in a [`.nowignore` configuration file](https://zeit.co/docs/v2/deployments/ignoring-source-paths).