Add .gitattributes file

This tells git to checkout all .md and .json files with Unix line feeds,
even on Windows. This shouldn't be a problem for working with these
files on Windows, as all modern text editors and IDEs should be capable
of editing Unix files transparently; but it makes it possible to run
`go generate ./...` on Windows, which assumes Unix line feeds in a few
places.

.gitattributes

@@ -0,0 +1,3 @@
+*.go    text
+*.md    text eol=lf
+*.json  text eol=lf

.gitignore

@@ -19,6 +19,7 @@ lazygit.exe
 
 # Exceptions
 !.gitignore
+!.gitattributes
 !.goreleaser.yml
 !.golangci.yml
 !.circleci/