Corrected the installation procedure (#849)

* Corrected the installation procedure

Now it can be put into a script.

* Updated the file tree

Necessary after installation instruction change

README.md

@@ -65,12 +65,15 @@ versions of build tools._
 $ git clone https://github.com/google/benchmark.git
 # Benchmark requires Google Test as a dependency. Add the source tree as a subdirectory.
 $ git clone https://github.com/google/googletest.git benchmark/googletest
+# Go to the library root directory
+$ cd benchmark
 # Make a build directory to place the build output.
 $ mkdir build && cd build
 # Generate a Makefile with cmake.
 # Use cmake -G <generator> to generate a different file type.
 $ cmake ../
 # Build the library.
+# Use make -j<number_of_parallel_jobs> to speed up the build process, e.g. make -j8 .
 $ make
 ```
 This builds the `benchmark` and `benchmark_main` libraries and tests.
@@ -78,12 +81,12 @@ On a unix system, the build directory should now look something like this:
 
 ```
 /benchmark
-/build
-  /src
-    /libbenchmark.a
-    /libbenchmark_main.a
-  /test
-    ...
+  /build
+    /src
+      /libbenchmark.a
+      /libbenchmark_main.a
+    /test
+      ...
 ```
 
 Next, you can run the tests to check the build.