update band math exercise

Author: Sara Safavi

jupyter-notebooks/in-class-exercises/band-math-generate-ndvi/generate-ndvi-exercise.ipynb

@@ -13,6 +13,9 @@
    "source": [
     "A **vegetation index** is generated by combining two or more spectral bands from a satellite image. There are many different vegetation indices; in this exercise we'll learn about the most commonly-used index.\n",
     "\n",
+    "### _How to use this exercise_\n",
+    "\n",
+    "_Adjacent to this notebook, you'll find a notebook titled `generate-ndvi-exercise-key.ipynb`: that notebook contains the \"answers\", while the code in *this* notebook is incomplete. As you work through this notebook, you'll see some codeblocks that contain `None` values: replace these `None` instances with your own code until the codeblocks here run successfully. If you get stuck try to use outside resources like documentation, previous work, or even other students here in this class before taking a peek at the Key._\n",
     "\n",
     "### NDVI\n",
     "Researchers often use a vegetation index called NDVI to measure the \"greenness\" or density of vegetation across a landscape. In addition to monitoring vegetation health, NDVI _(Normalized Difference Vegetation Index)_ can be used to track climate change, agricultural production, desertification, and land cover change. Developed by NASA scientist Compton Tucker in 1977, NDVI is derived from satellite imagery and compares reflected near-infrared light to reflected visible red light. It can be expressed as following equation:"