[docs] Improve assetcaching.md (#19625)

* Improve asset caching readme * Update assetcaching.md Co-authored-by: Robert Schumacher <roschuma@microsoft.com>
microsoft · Aug 18, 2021 · 6b24f0f · 6b24f0f
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diff --git a/docs/README.md b/docs/README.md
@@ -19,6 +19,7 @@ Vcpkg helps you manage C and C++ libraries on Windows, Linux and MacOS. This too
 - [Configuration and Environment](users/config-environment.md)
 - [Manifest Mode](users/manifests.md)
 - [Binary Caching](users/binarycaching.md)
+- [Asset Caching](users/assetcaching.md)
 - [Versioning](users/versioning.md)
 - [Usage with Android](users/android.md)
 - [Usage with Mingw-w64](users/mingw.md)

diff --git a/docs/users/assetcaching.md b/docs/users/assetcaching.md
@@ -31,7 +31,22 @@ Syntax: `x-azurl,<url>[,<sas>[,<rw>]]`
 
 Adds an Azure Blob Storage source, optionally using Shared Access Signature validation. URL should include the container
 path and be terminated with a trailing `/`. SAS, if defined, should be prefixed with a `?`. Non-Azure servers will also
-work if they respond to GET and PUT requests of the form: `<url><sha512><sas>`.
+work if they respond to GET and PUT requests of the form: `<url><sha512><sas>`. As an example, if you set
+`X_VCPKG_ASSET_SOURCES` to `x-azurl,https://mydomain.com/vcpkg/,token=abc123,readwrite` your server should respond to
+`GET` and `PUT` requests of the form `https://mydomain.com/vcpkg/<sha512>?token=abc123`.
+
+You can also use the filesystem (e.g. a network drive) via `file://` as asset cache. For example you then set
+`X_VCPKG_ASSET_SOURCES` to `x-azurl,file:///Z:/vcpkg/assetcache/,,readwrite` when you have a network folder mounted at
+`Z:/`.
+
+The workflow of this asset source is:
+
+1. Attemp to read from the mirror
+2. (If step 1 failed) Read from the original url
+3. (If step 2 succeeded) Write back to the mirror
+
+You can enable/disable steps 1 and 3 via the [`<rw>`](#valid-source-strings) specifier and you can disable step 2 via
+`x-block-origin` below.
 
 See also the [binary caching documentation for Azure Blob Storage](binarycaching.md#azure-blob-storage-experimental) for
 more information on how to set up an `x-azurl` source.