Add descriptions of results.csv columns that are missing in the docs

[ledell]

Some of the columns are missing a description (e.g. predict_duration) in the docs where we describe the results.csv output. Also https://github.com/openml/automlbenchmark#output should have a link to this result format documentation.

All currently available fixed columns (metric names excluded):

['id', 'task', 'framework', 'constraint', 'fold', 'type', 'result', 'metric', 'mode', 'version',
 'params', 'app_version', 'utc', 'duration', 'training_duration', 'predict_duration', 'models_count', 'seed', 'info']

Currently documented columns:

id
task
framework
fold
result
mode
version
params
tag -> more accurately replaced by app_version
utc
duration -> its meaning has changed
models -> replaced by model_count
seed
info

Must be added to doc:

constraint
type
result (update in relation with metric, as the result is now always in greater_is_better format)
metric
app_version
duration (update)
training_duration
predict_duration
models_count