Update cheatsheets

ppmdist

@@ -0,0 +1,17 @@
+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# ppmdist
+
+> Produce a grayscale version of a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ppmdist.html>.
+
+- Produce a grayscale version of the specified PPM image:
+
+`ppmdist {{path/to/input.ppm}} > {{path/to/output.pgm}}`
+
+- Use the specified method to map colors to graylevels:
+
+`ppmdist -{{frequency|intensity}} {{path/to/input.ppm}} > {{path/to/output.pgm}}`