Add README

internal/ccUnits/README.md

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+# ccUnits - A unit system for ClusterCockpit
+
+When working with metrics, the problem comes up that they may use different unit name but have the same unit in fact. There are a lot of real world examples like 'kB' and 'Kbyte'. In CC Metric Collector, the Collectors read data from different sources which may use different units or the programmer specifies a unit for a metric by hand. In order to enable unit comparison and conversion, the ccUnits package provides some helpers:
+
+There are basically two important functions:
+```go
+NewUnit(unit string) Unit
+GetUnitPrefixFactor(in, out Unit) (float64, error) // Get the prefix difference for conversion
+```
+
+In order to get the "normalized" string unit back, you can use:
+```go
+u := NewUnit("MB")
+fmt.Printf("Long string %q", u.String())
+fmt.Printf("Short string %q", u.Short())
+```
+
+If you have two units and need the conversion factor:
+```go
+u1 := NewUnit("kB")
+u2 := NewUnit("MBytes")
+factor, err := GetUnitPrefixFactor(u1, u2) // Returns an error if the units have different measures
+if err == nil {
+    v2 := v1 * factor
+}
+```
+
+If you have a metric and want the derivation to a bandwidth or events per second, you can use the original unit:
+
+```go
+in_unit, err := metric.GetMeta("unit")
+if err == nil {
+    value, ok := metric.GetField("value")
+    if ok {
+        out_unit = NewUnit(in_unit)
+        out_unit.AddDivisorUnit("seconds")
+        y, err := lp.New(metric.Name()+"_bw",
+                         metric.Tags(),
+                         metric.Meta(),
+                         map[string]interface{"value": value/time},
+                         metric.Time())
+        if err == nil {
+            y.AddMeta("unit", out_unit.Short())
+        }
+    }
+}
+```
+
+## Supported prefixes
+
+```go
+const (
+	Base  Prefix = iota
+	Peta         = 1e15
+	Tera         = 1e12
+	Giga         = 1e9
+	Mega         = 1e6
+	Kilo         = 1e3
+	Milli        = 1e-3
+	Micro        = 1e-6
+	Nano         = 1e-9
+	Kibi         = 1024
+	Mebi         = 1024 * 1024
+	Gibi         = 1024 * 1024 * 1024
+	Tebi         = 1024 * 1024 * 1024 * 1024
+)
+```
+
+The prefixes are detected using a regular expression `^([kKmMgGtTpP]?[i]?)(.*)` that splits the prefix from the measure. You probably don't need to deal with the prefixes in the code.
+
+## Supported measures
+
+```go
+const (
+	None Measure = iota
+	Bytes
+	Flops
+	Percentage
+	TemperatureC
+	TemperatureF
+	Rotation
+	Hertz
+	Time
+	Power
+	Energy
+	Cycles
+	Requests
+	Packets
+	Events
+)
+```
+
+There a regular expression for each of the measures like `^([bB][yY]?[tT]?[eE]?[sS]?)` for the `Bytes` measure.