Add note about extracting RTI provided openssl

[sloretz]

Relates to #662

[mikaelarguedas]

I recommend not merging this.

This is a duplicate of the information in the paragraph above and gives the wrong impression that RTI_OPENSSL_{LIBS,BIN} are used anywhere outside the test_security package.

The actual thing to do depends on the platform, it can be adding things to PATH, LD_LIBRARY_PATH, DYLD_LIBRARY_PATH ...

I would recommend removing that section and link people to official RTI documentation instead https://community.rti.com/static/documentation/connext-dds/5.3.1/doc/manuals/connext_dds/dds_security/RTI_SecurityPlugins_GettingStarted.pdf

[jacobperron]

I agree that this PR doesn't seem necessary.

[sloretz]

I recommend not merging this [...] I would recommend removing that section and link people to official RTI documentation instead

Roger. PR is now just adding a link to RTI's documentation.