merging in Leszek's changes to gh-pages

Thanks to @czterybity for this contribution, and to @wking for
mentoring this pull request.

Thanks to @r-gaia-cs, @mattphotonman, @gvwilson, @wltrimbl, and
 @ethanwhite for helpful commentary and feedback.

lessons/thw-python/strings-io/tutorial.ipynb

@@ -13,7 +13,7 @@
      "source": [
       "# Strings\n",
       "\n",
-      "**Prepared by Tommy Guy and Anthony Scopatz**\n",
+      "**Authors: Tommy Guy, Anthony Scopatz, Will Scopatz, Leszek Tarkowski**\n",
       "\n",
       "Lesson goals:\n",
       "\n",
@@ -519,6 +519,33 @@
       "each string in the list, add it yourself."
      ]
     },
+    {
+     "cell_type": "markdown",
+     "metadata": {},
+     "source": [
+      "## Context managers\n",
+      "\n",
+      "Closing a file is something often neglected in Python due to the fact that\n",
+      "it is usually done automatically at the end of a script by Python's garbage collector,\n",
+      "a part of the interpreter responsible for closing unused resources.\n",
+      "\n",
+      "In less trivial scenarios, a file should be closed after using it to prevent\n",
+      "data corruption. To ensure this, you can use a special language construct\n",
+      "called a context manager, available since Python 2.5.\n",
+      "\n",
+      "```python\n",
+      "   with open('outfile.txt','w') as f:\n",
+      "       f.write(\"Message of a Great Importance\")\n",
+      "\n",
+      "   #other instructions, file is already closed at this point\n",
+      "```\n",
+      "\n",
+      "Also called \"with statements\", context managers are responsible for releasing\n",
+      "resources when they are no longer needed.  In this example, the context manager\n",
+      "opens a file, creates a file handle variable to the open file named `f`, and after\n",
+      "the block of instructions completes, closes the file."
+     ]
+    },
     {
      "cell_type": "markdown",
      "metadata": {},