package: add git hook instructions

Add instructions and a hook to call core-validate-commit from
git commits.

Depends on nodejs#11

Known issues:
- leaves ${TMPF} around (temporary file)
- probably will reject `fixup:` type commit logs
- may be too strict if someone is just doing a simple
'work in progress' kind of local branch

Larger improvement:
- fold the entire hook script into a mode, and/or an alternate
script entrypoint into core-validate-commit. This would drop
requirements on sh, sed, grep…

Fixes: nodejs#12

README.md

@@ -31,6 +31,20 @@ $ core-validate-commit --list
  title-length enforce max length of commit title
 ```
 
+## Git hook installation
+
+- install `node` and `core-validate-commits` as above
+- install this hook in your clone of node (here `~/src/node`):
+```
+ln -s commit-msg.sh   ~/src/node/.git/hooks/commit-msg
+```
+- Alternatively, you may be able to pull from the npm-installed package:
+```
+ln  -s /usr/local/lib/node_modules/core-validate-commit/commit-msg.sh \
+          ~/src/node/.git/hooks/commit-msg
+```
+- make sure `~/src/node/.git/hooks/commit-msg` is executable.
+
 ## Test
 
 ```bash

commit-msg.sh

@@ -0,0 +1,70 @@
+#!/bin/sh
+
+# git hook to call core-validate-commits when any commit (or amend) is done
+
+# Author: Steven R. Loomis <srloomis@us.ibm.com>
+
+# Usage:  commit-msg .git/COMMIT_EDITMSG
+#  ( called by git )
+
+# 0. install node
+# 1. npm i -g core-validate-commits
+# 2. ln -s `pwd`/commit-msg.sh   ~/src/node/.git/hooks/commit-msg
+# ( You may be able to do
+#      ln  -s /usr/local/lib/node_modules/core-validate-commit/commit-msg.sh \
+#          ~/src/node/.git/hooks/commit-msg
+#  to pull from the npm installed version)
+# 3. make sure "commit-msg" is executable.
+
+#Debug:
+#pwd
+#echo COMMIT-MSG $*
+#env
+
+# We'll use this as a temporary file.
+# TODO: clean up after ourselves.
+TMPF=$(mktemp)
+
+#Debug:
+#echo Temporary file: ${TMPF}
+
+
+# We need to 'fix up' the commit message to emulate 'git show'
+# 1. header information
+# 2. 4 space indent
+# 3. strip comments
+
+# 1. add header
+cat >"${TMPF}" <<EOF
+commit 0
+Author: ${GIT_AUTHOR_NAME} <${GIT_AUTHOR_EMAIL}>
+Date:    Jan 1 00:00:00 1970 -0000
+
+EOF
+
+# 2/3. indent 4 spaces and remove comment lines.
+grep -v '^#' < "${1}" | sed -e 's%^.*$%    &%'  >> "${TMPF}"
+
+# by default, don't validate metadata
+OPTS="--no-validate-metadata"
+
+# Check which branch we're on
+BRANCH=$(git symbolic-ref HEAD)
+
+if [[ "${BRANCH}" == "refs/heads/master" ]];
+then
+    # we're on master - so be more careful
+    # do validate metadata
+    OPTS=""
+fi
+
+
+# Now, run! 
+
+#Debug:
+#set -x
+
+core-validate-commit ${OPTS} "file://${TMPF}" ||
+    ( echo "Please fix the above and try again. Your message is in ${1}" ; exit 1)
+# TODO: rm "${TMPF}"  - or perhaps trap?
+