doc: add make install to build-osx.md and build-unix.md

doc/build-osx.md

@@ -81,6 +81,10 @@ After exiting, you'll get a warning that the install is keg-only, which means it
 
         make check
 
+4.  (Optional) You can also install bitcoind to your path:
+
+        make install
+
 Creating a release build
 ------------------------
 You can ignore this section if you are building `bitcoind` for your own use.