Create CODEOWNERS (#323)

Author: jackdawm

.github/CODEOWNERS

@@ -0,0 +1,11 @@
+# These owners will be the default owners for everything in
+# the repo. Unless a later match takes precedence,
+# @temporalio/sdk will be requested for review when
+# someone opens a pull request.
+* @temporalio/sdk
+
+# Security team must approve exceptions to static analysis
+# scans, and any custom rules.
+
+.semgrepignore @temporalio/security
+.semgrep/ @temporalio/security