Fix 2015/burton (add missing file, apply patch)

On Dave Burton's web page he provided a patch with a bug fix in his
entry. This patch has been applied to prog.c (not the alt code).

Added the missing file spoilers.markdown (but changed the extension to
.md).

Format fixes in README.md. Didn't check the full file yet. That'll
almost certainly come another time in another pull request.

2015/burton/README.md

@@ -9,7 +9,7 @@
 make
 ```
 
-### To run:
+## To run:
 
 ```sh
 ./prog "expression"
@@ -23,7 +23,7 @@ or:
 
 Type in expressions on standard input.
 
-### Try:
+## Try:
 
 ```sh
 ./prog '?0x3343<<2'
@@ -49,7 +49,7 @@ grep '[0-9]' prog.c
 man ./calc.1
 ```
 
-## Alternate code
+## Alternate code:
 
 An alternate version of this entry, `prog.alt.c`, is provided.
 This alternate code is discussed in the "prog.c vs prog.alt.c" section of the
@@ -210,7 +210,7 @@ With -Wall -std=c99, both are quite noisy, and all can be ignored:
   * `value computed is not used [-Wunused-value]`
   * `operation on 'j' may be undefined [-Wsequence-point]`
 
-### Test Suite
+### Test Suite:
 
 If the program name begins with an "e", it echoes stdin to stdout.
 This allows for a convenient test suite:
@@ -220,9 +220,9 @@ ln -s prog eprog
 PATH=. eprog < test.in | diff - test.out
 ```
 
-### Spoilers
+### Spoilers:
 
-If you do not want to puzzle out how it works, see `spoilers.markdown`.
+If you do not want to puzzle out how it works, see [spoilers.md](spoilers.md).
 
 ## Copyright and CC BY-SA 4.0 License: