Exclude the external directory with a .clang-format file.

(I copied this trick from https://stackoverflow.com/a/57272592/1739415)

The GitHub clang-format workflow was already ignoring the `external`
directory because of the setting in the `clang-format.yml` file, but
that doesn't help other people running `clang-format` by hand (which the
CONTRIBUTING.md file instructs developers to do). Without this change,
I'd see a bunch of changes appear in the `external/spdlog` every time I
ran clang-format by hand.

I figure it's better to configure this via files that clang knows about
so both the GitHub workflow *and* developers have a closer to identical
experience.

.github/workflows/clang-format.yml

@@ -18,7 +18,6 @@ jobs:
     - uses: DoozyX/clang-format-lint-action@v0.9
       with:
         source: './src'
-        exclude: './external'
         extensions: 'h,cpp'
         clangFormatVersion: 11
         style: file

external/.clang-format

@@ -0,0 +1,2 @@
+DisableFormat: true
+SortIncludes: false