Update readme for optimizer and rewriter tools

README.md

@@ -142,6 +142,56 @@ result = Hardmax(v)
 
 More examples can be found in the [docs/examples](docs/examples) directory.
 
+## ONNX Script Tools
+
+### ONNX Optimizer
+
+The ONNX Script Optimizer tool provides the user with the functionality to optimize an ONNX model by performing optimizations and clean-ups such as constant folding, dead code elimination, etc. In order to utilize the optimizer tool:
+
+```python
+import onnxscript
+
+onnxscript.optimizer.optimize(onnx_model)
+```
+
+For a detailed summary of all the optimizations applied by the optimizer call, refer to the tutorial [Optimizing a Model using the Optimizer](https://onnxscript.ai/tutorial/optimizer/optimize.html)
+
+### ONNX Rewriter
+
+The ONNX Rewriter tool provides the user with the functionality to replace certain patterns in an ONNX graph with another pattern based on user-defined rewrite rules. The rewriter tools allows two different methods in which patterns in the graph can be rewritten.
+
+### Pattern-based rewriting
+
+For this style of rewriting, the user provides a `target_pattern` that is to be replaced, a `replacement_pattern` and a `match_condition` (pattern rewrite will occur only if the match condition is satisfied). A simple example on how to use the pattern-based rewriting tool is as follows:
+
+```python
+from onnxscript.rewriter import pattern
+
+# Create multiple rules
+rule1 = pattern.RewriteRule(
+    erf_gelu_pattern,  # Target Pattern
+    gelu,  # Replacement
+)
+rule2 = pattern.RewriteRule(
+    erf_gelu_pattern_2,  # Target Pattern
+    gelu,  # Replacement
+)
+# Create a Rewrite Rule Set with multiple rules.
+rewrite_rule_set = pattern.RewriteRuleSet([rule1, rule2])
+# Apply rewrites
+model_with_rewrite_applied = onnxscript.rewriter.rewrite(
+    model,  # Original ONNX Model
+    pattern_rewrite_rules=rewrite_rule_set,
+)
+return model_with_rewrite_applied
+```
+
+For a detailed tutorial on how to create target_pattern, replacement_pattern and match_condition blocks in order to utilize the pattern-based rewriter, refer to the tutorial [Pattern-based Rewrite Using Rules](https://onnxscript.ai/tutorial/rewriter/rewrite_patterns.html)
+
+### Function-based rewriting
+
+This style of rewriting matches a `FUNCTION_KEYWORD` and `PACKAGE_NAME` provided by the user to an existing function within the graph and replaces it with a new function provided by the user.
+
 ## Development Guidelines
 
 Every change impacting the converter or the eager evaluation must be