libarchive: Add README-CMake.txt

Describe how to update libarchive from upstream.

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+The Utilities/cmlibarchive directory contains a reduced distribution
+of the libarchive source tree with only the library source code and
+CMake build system.  It is not a submodule; the actual content is part
+of our source tree and changes can be made and committed directly.
+
+We update from upstream using Git's "subtree" merge strategy.  A
+special branch contains commits of upstream libarchive snapshots and
+nothing else.  No Git ref points explicitly to the head of this
+branch, but it is merged into our history.
+
+Update libarchive from upstream as follows.  Create a local branch to
+explicitly reference the upstream snapshot branch head:
+
+ git branch libarchive-upstream 2f4a3792
+
+Use a temporary directory to checkout the branch:
+
+ mkdir libarchive-tmp
+ cd libarchive-tmp
+ git init
+ git pull .. libarchive-upstream
+ rm -rf *
+
+Now place the (reduced) libarchive content in this directory.  See
+instructions shown by
+
+ git log 2f4a3792
+
+for help extracting the content from the upstream svn repo.  Then run
+the following commands to commit the new version.  Substitute the
+appropriate date and version number:
+
+ git add --all
+
+ GIT_AUTHOR_NAME='LibArchive Upstream' \
+ GIT_AUTHOR_EMAIL='libarchive-discuss@googlegroups.com' \
+ GIT_AUTHOR_DATE='2011-12-19 18:30:59 -0500' \
+ git commit -m 'libarchive 3.0.1-r3950 (reduced)' &&
+ git commit --amend
+
+Edit the commit message to describe the procedure used to obtain the
+content.  Then push the changes back up to the main local repository:
+
+ git push .. HEAD:libarchive-upstream
+ cd ..
+ rm -rf libarchive-tmp
+
+Create a topic in the main repository on which to perform the update:
+
+ git checkout -b update-libarchive master
+
+Merge the libarchive-upstream branch as a subtree:
+
+ git merge -s recursive -X subtree=Utilities/cmlibarchive \
+           libarchive-upstream
+
+If there are conflicts, resolve them and commit.  Build and test the
+tree.  Commit any additional changes needed to succeed.
+
+Finally, run
+
+ git rev-parse --short=8 libarchive-upstream
+
+to get the commit from which the libarchive-upstream branch must be started
+on the next update.  Edit the "git branch libarchive-upstream" line above to
+record it, and commit this file.