update README.md

Signed-off-by: Yi Wu <yiwu@pingcap.com>

README.md

-# titan
+# Titan: A RocksDB Plugin to Reduce Write Amplification
\ No newline at end of file
+
+Titan is RocksDB Plugin for key-value separation, inspired by 
+[WiscKey](https://www.usenix.org/system/files/conference/fast16/fast16-papers-lu.pdf).
+For introduction and design details, see our
+[blog post](https://pingcap.com/blog/titan-storage-engine-design-and-implementation/).
+
+## Build and Test
+Titan rely on RocksDB source code to build. You need to checkout RocksDB source code locally,
+and provide the path to Titan build script.
+```
+# To build:
+mkdir -p build
+cd build
+cmake .. -DROCKSDB_DIR=<rocksdb_source_dir>
+make -j<n>
+
+# Build static lib (i.e. libtitan.a) only:
+make titan -j<n>
+
+# Release build:
+cmake .. -DROCKSDB_DIR=<rocksdb_source_dir> -DCMAKE_BUILD_TYPE=Release
+
+# Building with sanitizer (e.g. ASAN):
+cmake .. -DROCKSDB_DIR=<rocksdb_source_dir> -DWITH_ASAN=ON
+
+# Building with compression libraries (e.g. snappy):
+cmake .. -DROCKSDB_DIR=<rocksdb_source_dir> -DWITH_SNAPPY=ON
+
+# Run tests after build. You need to filter tests by "titan" prefix.
+ctest -R titan
+```