Improve instructions to install formatter (#6162)

Summary:
While the instruction of installing "make format" dependencies works on some platforms, it is hard to use for some others. Improve it a little bit.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6162

Test Plan: Run "make format" on an envrionment missing the dependencies and see the instructions printed out

Differential Revision: D18970773

fbshipit-source-id: fd21b31053407cc171a6675f781a556a1c3e8945

build_tools/format-diff.sh

@@ -15,7 +15,12 @@ then
   echo "    curl --location http://goo.gl/iUW1u2 -o ${CLANG_FORMAT_DIFF}"
   echo "You can download clang-format by running:"
   echo "    brew install clang-format"
+  echo "  Or"
+  echo "    apt install clang-format"
+  echo "  This might work too:"
+  echo "    yum install git-clang-format"
   echo "Then, move both files (i.e. ${CLANG_FORMAT_DIFF} and clang-format) to some directory within PATH=${PATH}"
+  echo "and make sure ${CLANG_FORMAT_DIFF} is executable."
   exit 128
 fi