Clarify liboqs_DIR naming convention (open-quantum-safe#292)

* Clarify liboqs_DIR naming convention

As per discussion in open-quantum-safe#277 we want to clarify why the env var
`liboqs_DIR` does not follow standard naming convention by
using lowercase.

* added link to cmake command doc

CONFIGURE.md

@@ -36,6 +36,8 @@ This environment variable must be set to the location of the `liboqs` installati
 utilized in the build.
 By default, this is un-set, requiring installation of `liboqs` in a standard
 location for the OS.
+This uses the [`find_package`](https://cmake.org/cmake/help/latest/command/find_package.html)
+command in `cmake`, which checks for local builds of a package at `<PackageName>_DIR`
 
 ### USE_ENCODING_LIB