Update CODEOWNERS (#144)

.github/CODEOWNERS

@@ -1 +1,11 @@
-*       @wemeetagain @mpetrunic @tuyennhv @dapplion @mpetrun5
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# They will be requested for
+# review when someone opens a pull request.
+*       @ChainSafe/lodestar
+
+# Order is important; the last matching pattern takes the most
+# precedence. When someone opens a pull request that only
+# modifies md files, only md owners and not the global
+# owner(s) will be requested for a review.
+*.md    @ChainSafe/lodestar