Add partly written documentation for antlr parser

Author: anthonyyangdev

docs/GrammarParser.md

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+# Grammar Parser
+
+This documentation details the design and implementation of the Harmony programming language parser.
+
+- [Grammar Parser](#grammar-parser)
+  - [Modifying Harmony.g4](#modifying-harmonyg4)
+    - [Expressions](#expressions)
+    - [Statements](#statements)
+    - [Embedded Python Code in `Harmony.g4`](#embedded-python-code-in-harmonyg4)
+  - [Modifying `custom_denter.py`](#modifying-custom_denterpy)
+
+## Modifying Harmony.g4
+
+The `Harmony.g4` defines the grammar rules (statements and expressions) for the Harmony programming
+language written using `ANTLR 4.9.3`.
+
+> **NOTE**: The version of the Python3 antlr4 runtime must match
+the parser generator version exactly. That is, if the ANTLR parser generator version is change 
+to `ANTLR x.y.z`, then the `antlr4-python3-runtime==x.y.z` pip package has to be set as the
+compiler's dependency.
+
+If the `Harmony.g4` is modified, then run `make parser` to generate the updated `HarmonyLexer.py`,
+`HarmonyParser.py`, and `HarmonyVisitor.py` files. **Any changes to `HarmonyVisitor.py`, such as new/deleted/modified method headers must be reflected in the HarmonyVisitor implementation `antlr_rule_visitor.py`**.
+
+### Expressions
+
+### Statements
+
+### Embedded Python Code in `Harmony.g4`
+
+## Modifying `custom_denter.py`
+
+The `ModifiedDenterHelper` class implemented in this file extends the `DenterHelper` class of the
+`antlr-denter` dependency and overrides the internal behavior. The main behavior of interest is handling
+expressions that are broken into separate lines. Without modification, the default `DenterHelper` class would create `INDENT` and `DEDENT` tokens in between expression tokens, which would be difficult to parse using ANTLR 4. The extended implementation adds a check when the current token is a new indent while the previous observed token is an operator, such as `+` or `/`, which would be followed by another expression. If that's the case, then the observed `INDENT` (and the `DEDENT` token that would have also been output) is ignored.
+