Update README, specifically Windows section

[Ilia-Kosenkov]

The Windows section is particularly outdated providing recent developments in how we build on Windows.
(see #9 and #29).

I suggest to also include powershell commands as it is the new standard, plus default shell is cumbersome and sometimes difficult to navigate.
I tried to summarise Windows-related updates in libR-sys/readme-windows.
This can be merged as PR if it seems reasonable.

Also, README lacks description of feature flags and how to build bindgen vs pre-compiled versions.

[clauswilke]

Yes, please prepare a PR. Thanks!

[Ilia-Kosenkov]

PR #32 updated README, mostly Windows-specific part. Now a few more lines should be added to describe how to generate bindings on MacOS/Linux.

[clauswilke]

In particular, I just noticed that we never explicitly state how to build bindings from source:

cargo build --features use-bindgen

[Ilia-Kosenkov]

@clauswilke, I put that into Windows description, simple version suggests to use precomputed bindings, advanced -- to build your own using --features use-bindgen. As I did not touch other OS' sections, there is no mention of that, unfortuantely.

[Ilia-Kosenkov]

After discussion in extendr/rextendr#19, we need to revise the README, Windows section.
Right now the instructions are incorrect, at least for pre-computed bindings.
If we decided to use a single toolchain, stable-msvc, then to build for gnu targets we need linkers, which come from msys2.
So msys2 with mingw toolchains are required in any case.