Clarify the INSTALL instructions

Ensure users understand that they need to have appropriate permissions
to write to the install location.
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9268)

INSTALL

@@ -99,6 +99,9 @@
     $ nmake test
     $ nmake install
 
+ Note that in order to perform the install step above you need to have
+ appropriate permissions to write to the installation directory.
+
  If any of these steps fails, see section Installation in Detail below.
 
  This will build and install OpenSSL in the default location, which is:
@@ -108,6 +111,12 @@
            OpenSSL version number with underscores instead of periods.
   Windows: C:\Program Files\OpenSSL or C:\Program Files (x86)\OpenSSL
 
+ The installation directory should be appropriately protected to ensure
+ unprivileged users cannot make changes to OpenSSL binaries or files, or install
+ engines. If you already have a pre-installed version of OpenSSL as part of
+ your Operating System it is recommended that you do not overwrite the system
+ version and instead install to somewhere else.
+
  If you want to install it anywhere else, run config like this:
 
   On Unix:
@@ -941,8 +950,11 @@
        $ mms install                                    ! OpenVMS
        $ nmake install                                  # Windows
 
-     This will install all the software components in this directory
-     tree under PREFIX (the directory given with --prefix or its
+     Note that in order to perform the install step above you need to have
+     appropriate permissions to write to the installation directory.
+
+     The above commands will install all the software components in this
+     directory tree under PREFIX (the directory given with --prefix or its
      default):
 
        Unix:
@@ -998,6 +1010,12 @@
                         for private key files.
          misc           Various scripts.
 
+     The installation directory should be appropriately protected to ensure
+     unprivileged users cannot make changes to OpenSSL binaries or files, or
+     install engines. If you already have a pre-installed version of OpenSSL as
+     part of your Operating System it is recommended that you do not overwrite
+     the system version and instead install to somewhere else.
+
      Package builders who want to configure the library for standard
      locations, but have the package installed somewhere else so that
      it can easily be packaged, can use