Explicit remark about BUILD_SHARED_LIBS (libusb#349)

- a remark about using BUILD_SHARED_LIBS as a CACHE variable and CMP0077.

BUILD.cmake.md

@@ -179,6 +179,12 @@ set(BUILD_SHARED_LIBS FALSE) # HIDAPI as static library on all platforms
 add_subdirectory(hidapi)
 ```
 
+<details>
+  <summary>NOTE</summary>
+
+  If you project happen to use `BUILD_SHARED_LIBS` as a `CACHE` variable globally for you project, setting it as simple variable, as showed above _will have not affect_ up until _CMake 3.13_. See [CMP0077](https://cmake.org/cmake/help/latest/policy/CMP0077.html) for details.
+</details><br>
+
 There are several important differences in the behavior of HIDAPI CMake build system when CMake is built as standalone package vs subdirectory build:
 
 1) In _standalone build_ a number of standard and HIDAPI-specific variables are marked as _cache variables_ or _options_.