diff --git a/README.md b/README.md
index 7733dc0..65105f0 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,22 @@ were not carried over from the main manual.
 For general guidance on the documentation format, refer to the
 [contribution guidelines](https://doc.php.net/guide/contributing.md).
 
+## Creating this setup
+
+This manual builds on its own, independently of the main manual. The checkout has
+to be named `extensions`, as that is the directory `configure.php` is pointed at:
+
+```bash
+git clone https://github.com/php/doc-extensions extensions
+git clone https://github.com/php/doc-base
+php doc-base/configure.php --disable-libxml-check --with-base-lang=extensions
+```
+
+For general information related to creating this setup, see the
+[contribution guidelines](https://github.com/php/doc-base/blob/master/docs/contributing.md)
+or [this page](https://doc.php.net/tutorial/local-setup.php) on the documentation
+website.
+
 ## Building With make and Docker
 
 - Install Docker (https://docs.docker.com/get-docker/)
diff --git a/entities/entities.url.ent b/entities/entities.url.ent
new file mode 100644
index 0000000..3364311
--- /dev/null
+++ b/entities/entities.url.ent
@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="utf-8"?>
+<?do-not-translate?>
+<!--
+
+URLs, other addresses and external references.
+
+Cross-references whose target lives in the main manual go through the link
+entities below, as a linkend has no target in this manual.
+
+-->
+<entities xmlns       = "http://docbook.org/ns/docbook"
+          xmlns:xlink = "http://www.w3.org/1999/xlink"
+          translate   = "yes">
+
+<entity name="url.php.manual.en">https://www.php.net/manual/en/</entity>
+
+<entity name="link.book.libxml"><link
+ xlink:href="&url.php.manual.en;book.libxml.php">libxml</link></entity>
+
+<entity name="link.install.pecl"><link
+ xlink:href="&url.php.manual.en;install.pecl.php">Installation of PECL
+ extensions</link></entity>
+
+<entity name="link.install.windows.building"><link
+ xlink:href="&url.php.manual.en;install.windows.building.php">building
+ on Windows</link></entity>
+
+<entity name="link.ref.mcrypt"><link
+ xlink:href="&url.php.manual.en;ref.mcrypt.php">mcrypt</link></entity>
+
+<entity name="link.ref.mhash"><link
+ xlink:href="&url.php.manual.en;ref.mhash.php">mhash</link></entity>
+
+</entities>