speeding up build and adding installation instruction

README.md

@@ -39,8 +39,9 @@ sudo apt install graphviz doxygen build-essentials
 1. Build via cmake:
     ```
     cd <path_to_mantle_api>
-    cmake -B build -DCMAKE_BUILD_TYPE=Debug
-    cmake --build build
+    cmake -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_INSTALL_PREFIX=$PWD/install
+    cmake --build build -j
+    cmake --install build
     ```
 
 2. Build doc: