Update readme. Add build instructions (#69)

Author: Menooker

README.md

@@ -1,2 +1,66 @@
 # Graph Compiler
-Graph Compiler is in active development stage.
+Graph Compiler is an end-to-end, MLIR-based compiler designed to enhance the performance of deep learning workloads. It accepts computation graphs from the frontend, applies domain-specific optimizations and transformations, generates code, and manages runtime execution.
+
+The current frontend for Graph Compiler is [oneDNN Graph API](https://oneapi-src.github.io/oneDNN/graph_extension.html).
+
+## Build instructions
+
+### All-in-one compile script
+
+It is recommended for the users to use the all-in-one compile script at `scripts/compile.sh`. It downloads the LLVM dependency and builds the project.
+
+### Step-by-step build intructions
+
+To build this project step by step, first you need to find the LLVM commit-id we are using at `cmake/llvm-version.txt`. Then clone specific version of LLVM:
+
+```bash
+export LLVM_COMMIT_HASH=$(< cmake/llvm-version.txt)
+git clone https://github.com/llvm/llvm-project
+cd llvm-project
+git checkout $LLVM_COMMIT_HASH
+```
+
+Build LLVM with the command lines given in `.github/workflows/build-llvm.yml`:
+
+```bash
+mkdir llvm-install
+cmake -G Ninja llvm -B build -DCMAKE_INSTALL_PREFIX=llvm-install \
+  -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=true -DLLVM_ENABLE_PROJECTS="mlir" -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_INSTALL_UTILS=true -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -DLLVM_INSTALL_GTEST=ON
+cmake --build build --target install
+```
+
+Notes
+ * It is recommended to add optional options `-DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON` to the command `cmake -G Ninja llvm ...` above. These will enable the build of LLVM/MLIR dynamic libraries and let MLIR/LLVM tools link to them, to reduce the installed binary size of LLVM/MLIR. These options also enable the `GC_DEV_LINK_LLVM_DYLIB` option of graph-compiler repo (see below).
+ * The option `-DLLVM_INSTALL_GTEST=ON` is optional, if the tests of graph-compiler are disabled (see `GC_TEST_ENABLE` below).
+
+We have now installed LLVM at `llvm-project/llvm-install`.
+
+Change working directory to graph-compiler repo and prepare the build directory:
+
+```bash
+cd /PATH/TO/graph-compiler
+mkdir build && cd build
+```
+
+Build and run tests:
+
+```bash
+cmake .. -G Ninja \
+    -DCMAKE_BUILD_TYPE=RelWithDebInfo \
+    -DMLIR_DIR=/PATH/TO/llvm-project/llvm-install/lib/cmake/mlir \
+    -DLLVM_EXTERNAL_LIT=$(which lit)
+cmake --build . --target gc-check
+```
+
+Notes:
+ * `/PATH/TO/llvm-project/llvm-install` should be the install path of LLVM. If you installed LLVM elsewhere by `-DCMAKE_INSTALL_PREFIX` option when building LLVM, you need to change the path in `-DMLIR_DIR` accordingly.
+ *  The cmake option `-DLLVM_EXTERNAL_LIT` is for the tests of this project. It requires the `lit` tool to be installed in the system. You can install it via `pip install lit`. If you don't need to run the tests of this repo, you can omit this option in the command line.
+
+Graph Compiler supports the following build-time options.
+
+| CMake Option                    | Supported values (defaults in bold)    | Description                                                                            |
+|:--------------------------------|:---------------------------------------|:---------------------------------------------------------------------------------------|
+| GC_LEGACY_ENABLE                | **ON**, OFF                            | Controls building the legacy graph-compiler component                                  |
+| GC_TEST_ENABLE                  | **ON**, OFF                            | Controls building the tests                                                            |
+| GC_DEV_LINK_LLVM_DYLIB          | ON, **OFF**                            | Controls dynamic link LLVM/MLIR libraries, mainly for developer                        |
+