doc: add build-mlir2ncnn with commit sha and steps (#2706)

* doc: add build-mlir2ncnn with commit sha and steps

* add mlir git log detail

docs/how-to-build/build-mlir2ncnn.md

@@ -0,0 +1,56 @@
+# mlir2ncnn
+
+## Compile
+
+**Clone LLVM**
+```bash
+https://github.com/llvm/llvm-project.git
+git checkout -b mlir <a_working_commit_id>
+```
+Current working commit id is 7c15e0f64ccc79a53ed2db258f1cb58ec452a957:
+```
+$ git log
+
+commit 7c15e0f64ccc79a53ed2db258f1cb58ec452a957 (HEAD -> 01-26)
+Author: MaheshRavishankar <ravishankarm@google.com>
+Date:   Tue Jan 26 23:21:33 2021 -0800
+
+    [mlir][Linalg] Add canonicalization for init_tensor -> subtensor op.
+    
+    Differential Revision: https://reviews.llvm.org/D95305
+```
+
+It is determined by query lastest git commit date of `tools/mlir` directory.
+
+
+**Compile mlir**
+```bash
+cd llvm-project
+mkdir build
+cd build
+cmake -G Ninja -DCMAKE_INSTALL_PREFIX=install -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=ON -DLLVM_ENABLE_PROJECTS="mlir" -DLLVM_TARGETS_TO_BUILD="" -DLLVM_INCLUDE_EXAMPLES=OFF -DLLVM_INCLUDE_TESTS=OFF ../llvm/
+make -j8
+make install
+```
+
+**Compile mlir2ncnn**
+```bash
+cd tools/mlir
+mkdir build
+cd build
+cmake .. -D LLVM_DIR=<path/to/your/llvm_install/lib/cmake/llvm>
+make
+```
+
+## Usage
+
+**Export `.mlir`**
+
+See https://zhuanlan.zhihu.com/p/152535430
+
+
+**Usage mlir2ncnn**
+
+```
+./mlir2ncnn pix2pix.mlir pix2pix.param pix2pix.bin
+```