add caveat regarding OpenSSL installs [skip ci] (#402)

Signed-off-by: Michael Baentsch <57787676+baentsch@users.noreply.github.com>

USAGE.md

@@ -17,6 +17,18 @@ this, e.g., as follows:
 OpenSSL 3.2.0-dev  (Library: OpenSSL 3.2.0-dev )
 ```
 
+### Note on OpenSSL installation
+
+If one does not have an OpenSSL version installed in a suitable version, care
+is advised installing such version such as not to damage a pre-installed/system-wide
+`openssl` installation.
+
+In order to experiment with a local `openssl` version, we have made available
+[a shell script](scripts/fullbuild.sh) creating a local, non-system wide installed
+`openssl` binary. By default, the current "master" branch is built by this script
+but it can be configured to build any release/tag by setting the [OPENSSL_BRANCH](CONFIGURE.md#openssl_branch)
+environment variable.
+
 ## Activation
 
 Every OpenSSL provider needs to be activated for use. There are three main ways