SmallRyeConfigBuilderCustomizer documentation (#1029)

documentation/src/main/docs/config/customizer.md

@@ -0,0 +1,43 @@
+# Customizer
+
+A `SmallRyeConfigBuilderCustomizer` allows to customize a `SmallRyeConfigBuilder`, used to create the `SmallRyeConfig` 
+instance.
+
+Registration of a `SmallRyeConfigBuilderCustomizer` is done via the `ServiceLoader` mechanism by providing the
+implementation classes in a `META-INF/services/io.smallrye.config.SmallRyeConfigBuilderCustomizer` file. Alternatively, 
+customizers may be registered via the Programmatic API in `SmallRyeConfigBuilder#withCustomizers`.
+
+The `SmallRyeConfigBuilderCustomizer` may also
+assign a priority by overriding the default method `int priority()`. Customizers are sorted by ascending priority and 
+executed in that order, meaning that higher numeric priorities will execute last, possible overriding values set by 
+previous customizers.
+
+```java title="CustomConfigBuilder"
+package org.acme.config;
+
+import io.smallrye.config.SmallRyeConfigBuilder;
+import io.smallrye.config.SmallRyeConfigBuilderCustomizer;
+
+public class CustomConfigBuilder implements SmallRyeConfigBuilderCustomizer {
+    @Override
+    public void configBuilder(final SmallRyeConfigBuilder builder) {
+        builder.withDefaultValue("my.default", "1234");
+    }
+}
+```
+
+And registration in:
+
+```properties title="META-INF/services/io.smallrye.config.SmallRyeConfigBuilderCustomizer"
+org.acme.config.CustomConfigBuilder
+```
+
+The `CustomConfigBuilder` will be executed when creating a new `SmallRyeConfig` from a `SmallRyeConfigBuilder`:
+
+```java
+SmallRyeConfig config = new SmallRyeConfigBuilder().build();
+config.getValue("my.default", int.class);
+```
+
+A look up to `my.default` returns the value `1234`, registered by the `CustomConfigBuilder`.
+