add README.md for antlr

Author: Psilo

antlr4/README.md

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+# ANTLR 4 Grammar For RQL-Parsing
+
+In order to help write more concise and compact code when defining REST services, we use our own RQL-like parser to generate the queries for endpoints (using interceptors).
+
+## Development Setup
+
+You'll need VS Code to efficiently edit, compile and test the ANTLR Lexer and Parser.
+
+There is a `.vscode` directory in the `antlr4` folder containing the proper settings for VS Code.
+The `settings.json` contains settings for the ANTLR-plugin ([ANTLR4 grammar syntax support](https://marketplace.visualstudio.com/items?itemName=mike-lischke.vscode-antlr4)) which we use.
+The `launch.json` contains a debug-launch setting ready to be opened with `VS Code -> launch/debug -> open file`. Both contains variables you'll probably want to change:
+
+### settings.json
+
+`outputDir` is the directory (absolute) you want the parser-generator to copy the java-files into.
+
+`package` is the Java fully-qualified-package-name the files it generates should bear.
+
+### launch.json
+
+`input` the filename of the file that is used as input-file when debugging.
+
+`grammar` the grammar-file to start when hitting the debug-key.
+
+`startRule` the main-rule (root) of your grammar.
+
+### Usage
+
+Start up VS Code and open the folder.
+
+You may edit the grammar.
+
+When pressing `F5` a new tab should pop up with the title `Parse Tree: Rql.g4` showing the resulting parse-tree after parsing your grammar (see settings in `launch.json`).
+
+You may also go to your grammar file, right click and select either:
+
+- Show Railroad Diagram for Rule
+- Show Railroad Diagram for all Rules
+- Show ATN Graph for Rule
+- Show Grammar Call Graph
+
+Which will open in a new tab and are, depending on which point you chose,  depending on the rule your cursor currently is located in.
+
+After pressing `F5` your grammar is generated and the files are copied.
+
+Change to your Eclipse and refresh the folder the classes are located in, for Eclipse to detect and process the changes.
+
+You may use the visitor now and run some tests.