DOC: Added example of using OrderedDict for agg.
DOC: Added example of using OrderedDict for agg.

closes #12879

Author: Ben Kandel <bkandel@picwell.com>

Closes #13938 from bkandel/ordered_dict_example and squashes the following commits:

8097de8 [Ben Kandel] Changed to note.
0676294 [Ben Kandel] DOC: Added example of using OrderedDict for agg.

louispotok/pandas,amolkahat/pandas,datapythonista/pandas,jreback/pandas,pratapvardhan/pandas,dsm054/pandas,datapythonista/pandas,louispotok/pandas,zfrenchee/pandas,harisbal/pandas,louispotok/pandas,jreback/pandas,pandas-dev/pandas,kdebrab/pandas,toobaz/pandas,winklerand/pandas,linebp/pandas,gfyoung/pandas,harisbal/pandas,pandas-dev/pandas,linebp/pandas,kdebrab/pandas,cython-testbed/pandas,gfyoung/pandas,kdebrab/pandas,jmmease/pandas,DGrady/pandas,zfrenchee/pandas,Winand/pandas,cbertinato/pandas,dsm054/pandas,pratapvardhan/pandas,DGrady/pandas,linebp/pandas,linebp/pandas,winklerand/pandas,harisbal/pandas,jreback/pandas,TomAugspurger/pandas,pratapvardhan/pandas,DGrady/pandas,linebp/pandas,kdebrab/pandas,datapythonista/pandas,MJuddBooth/pandas,amolkahat/pandas,pratapvardhan/pandas,toobaz/pandas,jmmease/pandas,jorisvandenbossche/pandas,nmartensen/pandas,jmmease/pandas,nmartensen/pandas,nmartensen/pandas,GuessWhoSamFoo/pandas,jorisvandenbossche/pandas,winklerand/pandas,Winand/pandas,rs2/pandas,zfrenchee/pandas,MJuddBooth/pandas,zfrenchee/pandas,DGrady/pandas,TomAugspurger/pandas,jreback/pandas,amolkahat/pandas,rs2/pandas,Winand/pandas,jorisvandenbossche/pandas,zfrenchee/pandas,gfyoung/pandas,winklerand/pandas,cython-testbed/pandas,dsm054/pandas,MJuddBooth/pandas,MJuddBooth/pandas,cbertinato/pandas,cbertinato/pandas,jmmease/pandas,datapythonista/pandas,gfyoung/pandas,MJuddBooth/pandas,jmmease/pandas,jmmease/pandas,DGrady/pandas,nmartensen/pandas,rs2/pandas,jorisvandenbossche/pandas,cython-testbed/pandas,toobaz/pandas,cbertinato/pandas,pandas-dev/pandas,cbertinato/pandas,Winand/pandas,cython-testbed/pandas,amolkahat/pandas,cython-testbed/pandas,harisbal/pandas,winklerand/pandas,toobaz/pandas,gfyoung/pandas,GuessWhoSamFoo/pandas,DGrady/pandas,TomAugspurger/pandas,linebp/pandas,TomAugspurger/pandas,nmartensen/pandas,Winand/pandas,Winand/pandas,kdebrab/pandas,louispotok/pandas,GuessWhoSamFoo/pandas,dsm054/pandas,nmartensen/pandas,harisbal/pandas,pandas-dev/pandas,dsm054/pandas,jreback/pandas,pratapvardhan/pandas,toobaz/pandas,GuessWhoSamFoo/pandas,amolkahat/pandas,louispotok/pandas,rs2/pandas,GuessWhoSamFoo/pandas,winklerand/pandas