Add instructions for installing googlebenchmark (facebook#11282)

Summary:
Per the discussion in https://groups.google.com/g/rocksdb/c/JqhlvSs6ZEs/m/bnXZ7Q--AAAJ
It seems non-obvious that googlebenchmark must be installed manually before microbenchmarks can be run. I have added more detail to the installation instructions to make it clearer.

Pull Request resolved: facebook#11282

Reviewed By: cbi42

Differential Revision: D43874724

Pulled By: ajkr

fbshipit-source-id: f64a4ac4914cb057955d1ca965885f8822ca7764

INSTALL.md

@@ -48,6 +48,11 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
 * If you wish to build the RocksJava static target, then cmake is required for building Snappy.
 
 * If you wish to run microbench (e.g, `make microbench`, `make ribbon_bench` or `cmake -DWITH_BENCHMARK=1`), Google benchmark >= 1.6.0 is needed.
+* You can do the following to install Google benchmark. These commands are copied from `./build_tools/ubuntu20_image/Dockerfile`:
+
+`$ git clone --depth 1 --branch v1.7.0 https://github.com/google/benchmark.git ~/benchmark`
+
+`$ cd ~/benchmark && mkdir build && cd build && cmake .. -GNinja -DCMAKE_BUILD_TYPE=Release -DBENCHMARK_ENABLE_GTEST_TESTS=0 && ninja && ninja install`
 
 ## Supported platforms