Add instructions on how to build shared/dynamic library to the README

[Wandering-Consciousness]

I was trying to build the .NET (C#) wrapper project (liboqs-dotnet) and its README referenced this repo/README to build the oqs shared library but I noticed the README only mentioned that a static library was built. The Golang wrapper did have instructions on how to do so, so for consistency and benefit to future readers, I updated this repo's one too.

No

• [ no] Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• [ no] Does this PR change the the list of algorithms available -- either adding, removing, or renaming? (If so, PRs in OQS-OpenSSL, OQS-BoringSSL, and OQS-OpenSSH will also be required by the time this is merged.)

[baentsch]

Thanks for this suggestion: Makes good sense, as not everyone reads/finds the Wiki. Two suggestions for improvement, though: Could you begin section 3 with "By default [the main build result...]" and continue with your addition as written? Second, it may be a good idea to put a hyperlink for BUILD_SHARED_LIBS (to https://github.com/open-quantum-safe/liboqs/wiki/Customizing-liboqs#build_shared_libs). Otherwise LGTM.

[Wandering-Consciousness]

Thank you. I've made the changes you suggested.

[baentsch]

Thanks again. LGTM.