Add CONTRIBUTING.md (with checkpatch.sh examples)

CONTRIBUTING.md

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+# Contributing to the libevent
+
+## Coding style
+
+First and most generic rule: **just look around**.
+
+But, we have a script for checking patches/files/git-refs:
+```shell
+# Chech HEAD git ref
+./checkpatch.sh -r
+./checkpatch.sh -r HEAD
+
+# Check patch
+git format-patch --stdout -1 | ./checkpatch.sh -p
+git show -1 | ./checkpatch.sh -p
+
+# Or via regular files
+git format-patch --stdout -2
+./checkpatch.sh *.patch
+
+# Over a file
+./checkpatch.sh -d event.c
+./checkpatch.sh -d < event.c
+
+# And print the whole file not only summary
+./checkpatch.sh -f event.c
+./checkpatch.sh -f < event.c
+
+# See
+./checkpatch.sh -h
+```
+
+## Testing
+- Write new unit test in `test/regress_{MORE_SUITABLE_FOR_YOU}.c`
+- `make verify`