Add DEVELOPER.md with basic code stylistic guide

DEVELOPER.md

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+# Developer Guidelines
+
+## Golang code
+
+### Formatting
+
+Format your code using `go fmt -s`. Formatting of pushed code is automatically checked by GitHub action.
+
+### Style
+
+General guideline is to follow the code conventions of the surrounding code to keep the style within files consistent.
+
+The following rules are recommendations and are not required to be _always_ followed. Just use common sense to discern when to ignore a recommendation.
+
+#### YAML tags
+
+Services are configured using `.yaml` files, which are unmarshaled into a Golang structures on service start. Therefore there must be a strict correspondence between keys in `.yaml` files and YAML tags on the field of Golang configuration structure.
+
+For field names and YAML tags `camelCase` is used. Abbreviations and acronyms are all upper-case; except for tags at the beginning of a word when they should be all lower-case. Tag name should start with a lower-case letter, but otherwise should copy the field name.
+
+Example:
+
+```Go
+type Example struct {
+  Event     string        `yaml:"event"`
+  EventURL  string        `yaml:"eventURL"`
+  ExpiresIn time.Duration `yaml:"expiresIn"`
+  ID        string        `yaml:"id"`
+}
+```
+
+#### JSON tags
+
+Marshaling to and unmarshaling from JSON is used by structures that are part of public API. Therefore there might be a specification that defines how the marshaled fields must be named in a payload (for example: `subscriptionId`, `eventsUrl` from [OCF Cloud API for Cloud Services Specification](https://openconnectivity.org/specs/OCF_Cloud_API_For_Cloud_Services_Specification_v2.2.4.pdf)).
+
+For JSON tags strict `camelCase` is used, abbreviations and acronyms have the first letter upper-case and the rest is lower-case. Tag name should start with a lower-case letter and should usually copy the field name (this rule might be broken when a field is required to have a specific name).
+
+Example:
+
+```Go
+type Example struct {
+  Event     string        `json:"event"`
+  EventURL  string        `yaml:"eventUrl"`
+  ExpiresIn time.Duration `yaml:"expiresIn"`
+  ID        string        `yaml:"id"`
+}
+```