Added rule to ignore shared library file in .gitignore.

Also clarified README.txt in _clib.

Author: doodspav

.gitignore

@@ -127,3 +127,8 @@ dmypy.json
 
 # Pyre type checker
 .pyre/
+
+# Shared libraries
+src/atomics/_clib/*.dll*
+src/atomics/_clib/*.dylib*
+src/atomics/_clib/*.so*

src/atomics/_clib/README.txt

@@ -1,3 +1,5 @@
-This directory is meant to hold the patomic shared library (usually patomic.dll or libpatomic.so).
+This directory is meant to hold the patomic shared library (usually patomic.dll, libpatomic.dylib, libpatomic.so).
+This should automatically be built as a step in building a bdist_wheel or installing an sdist.
+On the off chance the build fails, the library can be built manually.
 The patomic source code can be found at https://github.com/doodspav/patomic (potentially only on the devel branch).
 Compilation with CMake should work painlessly (the code is ANSI compatible).