Add README to library-syntax module

library-syntax/README.md

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+# Markwon-syntax
+
+This is a simple module to add **syntax-highlight** functionality to your markdown rendered with Markwon library. It is based on [Prism4j](https://github.com/noties/Prism4j) so lead there to understand how to configure `Prism4j` instance.
+
+![theme-default](/art/markwon-syntax-default.png)
+
+![theme-darkula](/art/markwon-syntax-darkula.png)
+
+---
+
+First, we need to obtain an instance of `Prism4jSyntaxHighlight` which implements Markwon's `SyntaxHighlight`:
+
+```java
+final SyntaxHighlight highlight = 
+    Prism4jSyntaxHighlight.create(Prism4j, Prism4jTheme);
+```
+
+we also can obtain an instance of `Prism4jSyntaxHighlight` that has a _fallback_ option (if a language is not defined in `Prism4j` instance, fallback language can be used):
+
+```java
+final SyntaxHighlight highlight = 
+    Prism4jSyntaxHighlight.create(Prism4j, Prism4jTheme, String);
+```
+
+Generally obtaining a `Prism4j` instance is pretty easy:
+
+```java
+final Prism4j prism4j = new Prism4j(new GrammarLocatorDef());
+```
+
+Where `GrammarLocatorDef` is a generated grammar locator (if you use `prism4j-bundler` annotation processor)
+
+`Prism4jTheme` is a specific type that is defined in this module (`prism4j` doesn't know anything about rendering). It has 2 implementations:
+
+* `Prism4jThemeDefault`
+* `Prism4jThemeDarkula`
+
+Both of them can be obtained via factory method `create`:
+
+* `Prism4jThemeDefault.create()`
+* `Prism4jThemeDarkula.create()`
+
+But of cause nothing is stopping you from defining your own theme:
+
+```java
+public interface Prism4jTheme {
+
+    @ColorInt
+    int background();
+
+    @ColorInt
+    int textColor();
+
+    void apply(
+            @NonNull String language,
+            @NonNull Prism4j.Syntax syntax,
+            @NonNull SpannableStringBuilder builder,
+            int start,
+            int end
+    );
+}
+```
+
+> You can extend `Prism4jThemeBase` which has some helper methods