add protobuf installation as requirement

remove cmake patch for abseil/protobuf

README.md

@@ -2,6 +2,25 @@
 
 ## Build
 
+### Protobuf
+
+Use the following commands to install protobuf:
+```
+git clone https://github.com/protocolbuffers/protobuf
+cd protobuf
+mkdir build
+cd build
+cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release
+ninja
+sudo ninja install
+```
+
+You can now remove the protobuf repo directory with:
+```
+cd ../..
+rm -rf protobuf
+```
+
 ### Mlir
 
 Follow the first part of instructions [here](onnx-mlir/docs/BuildOnLinuxOSX.md) to build mlir.