adds logos (#147)

assets/logos/README.md

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+# NWChemEx Branding Images (Logos for short)
+
+This directory contains images used to define the NWChemEx and NWChemEx
+Community brands.
+
+# Usage
+
+To link to an image in a markdown file (such as the NWChemEx Community website
+or a repo's README) do:
+
+```
+![alt text](https://github.com/NWChemEx/.github/blob/master/assets/logos/xxx)
+```
+
+where `xxx` should be replaced with the directory and the image you want. Note
+that GitHub will only display PNG and JPEG images.
+
+AFAIK you must make a copy of an image to link to it in reStructuredText, i.e.,
+you can't embed an image stored in a different repo.
+
+# Directory Layout
+
+The NWChemEx logos are grouped into four categories:
+
+1. Badges - Used to display one's role in the NWChemEx Community (TODO: link to
+   community roles).
+2. Banners - Meant to be used as a splashy representation of NWChemEx, e.g.,
+   as banners at the top of web pages, or in the acknowledgements of a slide
+   deck (when space is limited we instead recommend the full color logo).
+3. Full Color Logos - These are just the word "NWChemEx" stylized so that the
+   "NWChem" is one color and the "Ex" is another. You're welcome to use these
+   logos on their own, but they're primarily meant to be used as pieces of the
+   badges and banners.
+4. Mono Color Logos - Similar to the full color logos, but "NWChemEx" is a
+   single color. These logos for applications where
+   the name may be rendered in black and white.
+
+For reference we also have the CMYK and Pantone (PMS) codes for the colors.
+
+## Notes on File Formats
+
+Each logo is available in three file formats: JPG, PNG, and EPS. EPS is a
+vector format and recommended when you are going to be significantly enlarging
+the logos (for example on posters), particularly when the logo is going to be
+printed out. EPS files are large and their usage in websites etc. may inhibit
+performance. For virtual branding we recommend the PNG images (or the JPG if
+the PNG is still too large). Note that JPG and EPS are NOT vector formats and
+thus they should be used at their original size (or smaller) to avoid
+pixelation (n.b., PNG usually supports some enlarging without noticeable pixelation).
+
+# Acknowledgements
+
+Logos were designed by Grant Luhmann at Ames National Laboratory. The NWChemEx
+team is thankful for the time and effort Grant spent preparing these awesome
+images.