add howto doc

doc/howto.md

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+# How Tos
+
+## Concepts
+
+- `baseline`, one of the best KPI records in the history, the baseline should keep being updated.
+- `Factor`, alias for KPI to make code more readable.
+
+## Philosophy
+
+`MacroCE` is a highly customizable framework which 
+
+- triggers the execution of every evaluation and compare the KPIs with baseline,
+- helps to maintain the latest baseline,
+- displays the evaluation status or human-readable reports with a web page, and give an alert by sending emails if the evaluation of some version fails.
+
+It doesn't
+
+- care about the details about the evalution, in other words, one can embed any logics into a evaluation.
+
+
+## Adding new KPIs
+
+1. Reference [core.py](https://github.com/PaddlePaddle/MacroCE/blob/master/core.py), which defines the KPI base class called `Factor`.
+2. Inherit `Factor` and implement all the `UnImplemented` methods, read the class `GreaterWorseFactor`.
+3. Start a PR and merge the new KPI.
+
+## Adding new evaluations
+
+The baseline repo is at [baseline repo](https://github.com/Superjomn/paddle-modelci-baseline).
+
+Let's take the existing evaluation [resnet30](https://github.com/Superjomn/paddle-modelci-baseline/tree/master/resnet30) 
+for example, the files required by `MacroCE` are
+
+- `train.xsh`, it defines how to run this evaluation and generate the KPI records,
+- `continuous_evaluation.py`, it tells `MacroCE` the KPI Factors this evaluation uses.
+- `history/`, this is a directory and includes the baseline records.
+
+The `train.xsh` is just a script and nothing to do with MacroCE, so one can embed any logics in it, 
+for example, any logics including data preparation, or even request a remote service.
+
+The `continuous_evaluation.py` of `resnet30` looks like this
+
+```python
+import os
+import sys
+sys.path.append(os.environ['modelci_root'])
+from core import CostFactor, DurationFactor
+
+train_cost_factor = CostFactor('train_cost', 0.15)
+train_duration_factor = CostFactor('train_duration', 0.15)
+
+tracking_factors = [
+    train_cost_factor,
+    train_duration_factor,
+]
+```
+
+it creates two instances of KPI Factors `train_cost_factor` and `train_duration_factor`,
+both are defined in `MacroCE/core.py`. 
+The list `tracking_factors` is required by `MacroCE` which tells the KPIs this evaluation want to evaluate.
+
+Into the details of `train.xsh`, the `resnet30/model.py` defines some details of running the model (MacroCE
+do not care those logics, but the `train.xsh` need to add records to the KPI Factors in some way).
+
+Some code snippets related to adding KPI records are as follows
+
+```python
+# import KPI Factor instances
+from continuous_evaluation import (train_cost_factor,
+                                   train_duration_factor,
+                                   tracking_factors)
+# ...
+
+for batch_id, data in enumerate(train_reader()):
+    # ... train the model
+
+    # add factor record
+    train_cost_factor.add_record(np.array(avg_cost_, dtype='float32'))
+    train_duration_factor.add_record(batch_end - batch_start)
+
+
+# when the execution ends, persist all the factors to to files.
+for factor in tracking_factors:
+    factor.persist()
+```