[docs] Update README.md for brief descriptions of optimizer and rewri…

…ter tools. (#1702)

#TODO Add more details about function-based rewriting once tutorial for
function-based rewriting is merged

README.md

@@ -15,6 +15,16 @@ models using a subset of Python. ONNX Script is:
 * **Debuggable:** allows for eager-mode evaluation that provides for a
   more delightful ONNX model debugging experience.
 
+This repo also covers:
+
+* **ONNX IR:** an in-memory IR that supports the full ONNX spec, designed
+  for graph construction, analysis and transformation.
+* **ONNX Script Optimizer:** provides functionality to optimize an ONNX
+  model by performing optimizations and clean-ups such as constant folding,
+  dead code elimination, etc.
+* **ONNX Rewriter:** provides functionality to replace certain patterns in
+  an ONNX graph with replacement patterns based on user-defined rewrite rules.
+
 Note however that ONNX Script does **not** intend to support the entirety
 of the Python language.
 
@@ -142,6 +152,85 @@ result = Hardmax(v)
 
 More examples can be found in the [docs/examples](docs/examples) directory.
 
+## ONNX IR
+
+An in-memory IR that supports the full ONNX spec, designed for graph construction, analysis and transformation.
+
+### Features
+
+* **Full ONNX spec support:** all valid models representable by ONNX protobuf,
+  and a subset of invalid models (so you can load and fix them).
+* **Low memory footprint:** mmap'ed external tensors; unified interface for
+  ONNX TensorProto, Numpy arrays and PyTorch Tensors etc. No tensor size
+  limitation. Zero copies.
+* **Straightforward access patterns:** Access value information and traverse the
+  graph topology at ease.
+* **Robust mutation:** Create as many iterators as you like on the graph while mutating it.
+* **Speed:** Performant graph manipulation, serialization/deserialization to Protobuf.
+* **Pythonic and familiar APIs:** Classes define Pythonic apis and still map to
+  ONNX protobuf concepts in an intuitive way.
+
+## 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
+
+# The target pattern
+def erf_gelu_pattern(op, x):
+    return 0.5 * (x * (op.Erf(x / math.sqrt(2)) + 1.0))
+
+def erf_gelu_pattern_2(op, x):
+    return (x * (op.Erf(x / math.sqrt(2)) + 1.0)) * 0.5
+
+# The replacement pattern
+def gelu(op, x: ir.Value):
+    return op.Gelu(x, domain="com.microsoft")
+
+# 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