diff --git a/docs/internals.md b/docs/internals.md
new file mode 100644
index 0000000..b715d9f
--- /dev/null
+++ b/docs/internals.md
@@ -0,0 +1,46 @@
+# Internals
+
+## Unit testing
+
+The package is tested with [PHPUnit](https://phpunit.de/). To run tests:
+
+```shell
+./vendor/bin/phpunit
+```
+
+## Mutation testing
+
+The package tests are checked with [Infection](https://infection.github.io/) mutation framework with
+[Infection Static Analysis Plugin](https://github.com/Roave/infection-static-analysis-plugin). To run it:
+
+```shell
+./vendor/bin/roave-infection-static-analysis-plugin
+```
+
+## Static analysis
+
+The code is statically analyzed with [Psalm](https://psalm.dev/). To run static analysis:
+
+```shell
+./vendor/bin/psalm
+```
+
+## Rector
+
+Use [Rector](https://github.com/rectorphp/rector) to make codebase follow some specific rules or
+use either newest or any specific version of PHP:
+
+```shell
+./vendor/bin/rector
+```
+
+## Dependencies
+
+Use [ComposerRequireChecker](https://github.com/maglnet/ComposerRequireChecker) to detect transitive
+[Composer](https://getcomposer.org/) dependencies.
+
+To run the checker, execute the following command:
+
+```shell
+./vendor/bin/composer-require-checker
+```
diff --git a/psalm.xml b/psalm.xml
index 277e73d..500a864 100644
--- a/psalm.xml
+++ b/psalm.xml
@@ -3,7 +3,7 @@
     errorLevel="1"
     findUnusedBaselineEntry="true"
     findUnusedCode="false"
-    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
     xmlns="https://getpsalm.org/schema/config"
     xsi:schemaLocation="https://getpsalm.org/schema/config vendor/vimeo/psalm/config.xsd"
 >