Add some notes about missing/disabled cppcoreguildlines clang-tidy checkers

.clang-tidy

@@ -2,6 +2,8 @@ Checks: [-*,
          bugprone-*,
          performance-*,
 
+         # Enable a very limited number of the cppcoreguidelines checkers.
+         # See the notes for some of the rest of them below.
          cppcoreguidelines-macro-usage,
          cppcoreguidelines-misleading-capture-default-by-value,
          cppcoreguidelines-virtual-class-destructor,
@@ -33,4 +35,14 @@ Checks: [-*,
          # This one returns a bunch of findings in DFA and the sqlite library.
          # We're unlikely to fix either of them.
          -performance-no-int-to-ptr,
+
+         # These cppcoreguidelines checkers are things we should investigate
+         # and possibly fix, but there are so many findings that we're holding
+         # off doing it for now.
+         #cppcoreguidelines-init-variables,
+         #cppcoreguidelines-prefer-member-initializer,
+         #cppcoreguidelines-pro-type-member-init,
+         #cppcoreguidelines-pro-type-cstyle-cast,
+         #cppcoreguidelines-pro-type-static-cast-downcast,
+         #cppcoreguidelines-special-member-functions,
 ]