doc: Describe FIPSDIR environment variable

As per the OpenSSL User Guide, it is possible to use the FIPSDIR
environment variable to specify a custom install path for the
validated cryptographic module.

PR-URL: #3752
Reviewed-By: Michael Dawsson <michael_dawson@ca.ibm.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>
Reviewed-By: Fedor Indutny <fedor@indutny.com>

README.md

@@ -329,7 +329,9 @@ Instructions:
    Appendix A in the [security policy]
    (http://csrc.nist.gov/groups/STM/cmvp/documents/140-1/140sp/140sp1747.pdf).
    The only exception is that `./config no-asm` can be
-   used in place of `./config` )
+   used in place of `./config`, and the FIPSDIR environment variable
+   may be used to specify a non-standard install folder for the
+   validated module, as per User Guide sections 4.2.1, 4.2.2, and 4.2.3.
 6. Get into Node.js checkout folder
 7. `./configure --openssl-fips=/path/to/openssl-fips/installdir`
    For example on ubuntu 12 the installation directory was