potentially outdated INSTALL.md?

Hello

I just tried to compile latest version (6.17.3) following instructions on https://github.com/facebook/rocksdb/blob/master/INSTALL.md.
It wasn't very clear to me what is the right sequence of steps to compile optimized (production) shared object. I believe the first step is to generate make files with `cmake` like:

```
mkdir build && cd build &&
cmake .. -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/tmp/install_dir
```
and then
```
make -j16 rocksdb-shared && make -j16 install
```

Please note that I used `rocksdb-shared ` target as opposite to `lib_shared` that is mentioned in the document. `lib_shared` seems not exist in the `Makefile` that was generated by the `cmake`.

The document says by default the generated `.so` is not optimized. ` -DCMAKE_BUILD_TYPE=Release` would be enough to trigger the optimized build?

I also run into issue with `pthread` which I had to modify the root level `CMakeLists.txt`
```
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pthread")
```

If you think these steps are correct and current document is outdated I am happy to make a PR.

Let me know what you think

Thanks
Bart