add Readme.md for building and brief introduce

Readme.md

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+# XiangShan Open Verify Class-00-BPU
+
+This is the first class of XiangShan Open Verify. In this class, we will verify the BPU of XiangShan.  
+The repository is used to build the verification environment and testcases for the BPU.
+
+## Environment
+
+Install the following tools before running the testcases.
+
+1. [Verilator](https://www.veripool.org/projects/verilator/wiki/Installing)
+2. [Picker](https://github.com/XS-MLVP/picker)
+
+## Build
+
+### uFTB
+
+```bash
+make uftb LANG=python
+```
+
+Then you will see the following files in the `out` directory. The whole directory is the python module of uFTB of XiangShan.  
+
+```bash
+out
+`-- picker_out_uFTB
+    `-- UT_FauFTB
+        |-- _UT_FauFTB.so
+        |-- __init__.py
+        |-- libDPIFauFTB.a
+        |-- libUTFauFTB.so
+        |-- libUT_FauFTB.py
+        |-- uFTB.fst.hier
+        `-- xspcomm
+            |-- __init__.py
+            |-- __pycache__
+            |   |-- __init__.cpython-38.pyc
+            |   `-- pyxspcomm.cpython-38.pyc
+            |-- _pyxspcomm.so -> _pyxspcomm.so.0.0.1
+            |-- _pyxspcomm.so.0.0.1
+            |-- info.py
+            `-- pyxspcomm.py
+
+4 directories, 13 files
+```
+The `UT_FauFTB` is the python module of uFTB of XiangShan which can be imported in the testcases. For example, you can import the module by the following code when your python file is in the `picker_out_uFTB` directory.
+
+```python
+from UT_FauFTB import *
+
+if __name__ == "__main__":
+    uFTB = DUTFauFTB()
+    dut.init_clock("clk")
+
+    # Your testcases here
+
+    uFTB.finalize()
+```
+
+The other modules such as Tage_SC are similar to uFTB. You can import them in the same way.
+
+### Tage SC
+
+```bash
+make tage_sc LANG=python
+```
+
+### FTB
+
+```bash
+make ftb LANG=python
+```