Create README.md for test file

testfile/README.md

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+# Example coffea file 
+
+Quick tutorial to go through coffea files from commissioning framework.
+
+## Structure of the file
+
+The coffea contains histograms  wrapped in a dictionary with `$dataset:{$histname:hist}`, the `hist` is the object using
+[hist](https://hist.readthedocs.io/en/latest/) which allows multidimensional bins with different types of array
+```python
+{'WW_TuneCP5_13p6TeV-pythia8':{
+'btagDeepFlavB_b_0': Hist(
+  IntCategory([0, 1, 4, 5, 6], name='flav', label='Genflavour'),
+  IntCategory([1, -1], name='osss', label='OS(+)/SS(-)'),
+  StrCategory(['noSF'], growth=True, name='syst'),
+  Regular(30, -0.2, 1, name='discr', label='btagDeepFlavB_b'),
+  storage=Weight()) # Sum: WeightedSum(value=140, variance=140), 'btagDeepFlavB_bb_0': Hist(
+  IntCategory([0, 1, 4, 5, 6], name='flav', label='Genflavour'),
+  IntCategory([1, -1], name='osss', label='OS(+)/SS(-)'),
+  StrCategory(['noSF'], growth=True, name='syst'),
+  Regular(30, -0.2, 1, name='discr', label='btagDeepFlavB_bb'),
+  storage=Weight()) # Sum: WeightedSum(value=140, variance=140), 'btagDeepFlavB_lepb_0': Hist(
+  IntCategory([0, 1, 4, 5, 6], name='flav', label='Genflavour'),
+  IntCategory([1, -1], name='osss', label='OS(+)/SS(-)'),
+  StrCategory(['noSF'], growth=True, name='syst'),
+  Regular(30, -0.2, 1, name='discr', label='btagDeepFlavB_lepb'),
+  storage=Weight()) # Sum: WeightedSum(value=140, variance=140)}}
+```
+
+The histogram is a multidimentinal histogram, with all the axis listed
+```python
+Hist(
+  IntCategory([0, 1, 4, 5, 6], name='flav', label='Genflavour'),# different genflavor, 0 for light, 1 for PU, 2 for c, 3 for b. Always 0 for data.
+  IntCategory([1, -1], name='osss', label='OS(+)/SS(-)'),# opposite sign or same sign, only appears in W+c workflow
+  StrCategory(['noSF','PUUp','PUDown'], growth=True, name='syst'),# systematics variations,
+  Regular(30, -0.2, 1, name='discr', label='btagDeepFlavB_lepb'),# discriminator distribution, the last axis is always the variable
+  storage=Weight()) # Sum: WeightedSum(value=140, variance=140)# Value is sum of the entries, Variances is sum of the variances.
+```
+
+## Read coffea files and explore the histogram
+
+```python
+from coffea.util import load
+# open coffea file
+output=load("hists_ctag_Wc_sf_VV.coffea"）
+# get the histogram and read the info
+hist=output['WW_TuneCP5_13p6TeV-pythia8']['btagDeepFlavB_lepb_0']
+# addition for two histogram is possible if the axis is the same
+histvv=output['WW_TuneCP5_13p6TeV-pythia8']['btagDeepFlavB_lepb_0']+
+       output['WZ_TuneCP5_13p6TeV-pythia8']['btagDeepFlavB_lepb_0']+
+       output['ZZ_TuneCP5_13p6TeV-pythia8']['btagDeepFlavB_lepb_0']
+# To get 1D histogram, we need to reduce the dimention
+# we can specify the axis we want to read, e.g. read charm jet, opposite sign events with noSF
+axis={'flav':3,'os':0,'syst':'noSF'}
+hist1d=hist[axis]
+# you can also sum over the axis, e.g. here shows no jet flavor split and sum os+ss
+axis={'flav':sum,'os':sum,'syst':'noSF'}
+# rebin the axis is also possible, rebin the discrimnator by merged two bins into one
+axis={'flav':sum,'os':sum,'syst':'noSF','discr':hist.rebin(2)}
+```