Added README.

README.md

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+The CMAKE Tutorial
+------------------
+
+This is just the hand-coded result of following
+[The CMake Tutorial](https://cmake.org/cmake-tutorial/).  I've
+extensively commented my <code>CMakeLists.txt</code> files so that if I
+need to review the material, I'll have my own (hopefully sensible) words
+to guide my understanding.  I expect to be doing a lot more C/C++ in the
+coming months, and this was one of my starting points.
+
+Status
+------
+
+This project is **completed**.  No new work is being considered.
+
+Using
+-----
+
+Do the tutorial instead.  The only thing I had to look up was Newton's
+square root algorithm; I ended up using the one out of
+[SICP](https://mitpress.mit.edu/sicp/full-text/book/book.html), as you
+might be able to guess from the variable names.
+
+License
+-------
+
+The tutorial is under a Creative Commons CC 4.0/BY.  Works for me:
+
+> You are free to:
+>
+>Share — copy and redistribute the material in any medium or format
+>
+>Adapt — remix, transform, and build upon the material
+>for any purpose, even commercially.
+>
+>The licensor cannot revoke these freedoms as long as you follow the
+>license terms.
+>
+>Attribution — You must give appropriate credit, provide a link to the
+>license, and indicate if changes were made. You may do so in any
+>reasonable manner, but not in any way that suggests the licensor
+>endorses you or your use.