Update README.md

Signed-off-by: Reinhard Naegele <unguiculus@gmail.com>

README.md

@@ -1,3 +1,102 @@
 # gotf
 
-Terraform wrapper WIP
+`gotf` is a Terraform wrapper thst makes it easy to support multiple configuration, e. g. for different environments.
+
+## Installation
+
+### GitHub Release
+
+Download a release from GitHub:
+
+https://github.com/unguiculus/gotf/releases
+
+### Using `go get`
+
+```console
+$ go get github.com/unguiculus/gotf
+```
+
+## Usage
+
+```console
+$ gotf --help
+gotf is a Terraform wrapper facilitating configurations for various environments
+
+Usage:
+  gotf [flags] [Terraform args]
+
+Flags:
+  -c, --config string   Config file to be used (default "gotf.yaml")
+  -h, --help            help for gotf
+  -p, --params map      Params for templating in the config file. May be specified multiple times (default map[])
+      --version         version for gotf
+```
+
+`goft` is configured via a config file.
+By default, `gotf.yaml` is loaded from the current directory.
+
+### Example
+
+```yaml
+varsFiles:
+  # tfvars files are added to the Terraform environment via
+  # TF_CLI_ARGS_<command>=-var-file=<file> for commands that support them
+  - testmodule/test.tfvars
+vars:
+  # Variables are added to the Terraform environment via
+  # TF_VAR_<var>=value for commands that support them
+  foo: foovalue
+  templatedVar: "{{ .Params.param }}"
+envs:
+  # Environment variables are added to the Terraform calls environment
+  BAR: barvalue
+  TEMPLATED_ENV: "{{ .Params.param }}"
+backendConfigs:
+  # Backend configs are always added as variables (TF_VAR_<var>=value) for commands
+  # that support them and, if in case of 'init' additionally as '-backend-config' CLI options
+  backend_key: be_key_{{ .Vars.foo }}_{{ .Envs.BAR }}_{{ .Vars.templatedVar }}_{{ .Params.key_suffix }}
+  backend_storage_account_name: be_storage_account_name_{{ .Vars.foo }}_{{ .Envs.BAR }}
+  backend_resource_group_name: be_resource_group_name_{{ .Vars.foo }}_{{ .Envs.BAR }}
+  backend_container_name: be_container_name_{{ .Vars.foo }}_{{ .Envs.BAR }}
+```
+
+## Templating
+
+Go templating can be used in the config file as follows.
+
+* In the first templating pass, `vars` and `envs` are processed.
+  All parameters specified using the `-p|--param` flag are available in the `.Params` object.
+* In the second templating pass, `backendConfigs` are processed.
+  `vars` are available as `.Vars`, `envs` are available as `.Envs` with the results from the first templating pass.
+  Additionally, `.Params` is also available again.
+
+Using the above config file, running `terraform init` could look like this:
+
+```console
+$ gotf -c example-config.yaml -p param=myval -p key_suffix=mysuffix init
+```
+
+After processing, the config file would look like this:
+
+```yaml
+varsFiles:
+  # tfvars files are added to the Terraform environment via
+  # TF_CLI_ARGS_<command>=-var-file=<file> for commands that support them
+  - testmodule/test.tfvars
+vars:
+  # Variables are added to the Terraform environment via
+  # TF_VAR_<var>=value for commands that support them
+  foo: foovalue
+  templatedVar: "myval"
+envs:
+  # Environment variables are added to the Terraform calls environment
+  BAR: barvalue
+  TEMPLATED_ENV: "myval"
+backendConfigs:
+  # Backend configs are always added as variables (TF_VAR_<var>=value) for commands
+  # that support them and, if in case of 'init' additionally as '-backend-config' CLI options
+  backend_key: be_key_foovalue_barvalue_myval_mysuffix
+  backend_storage_account_name: be_storage_account_name_foovalue_barvalue
+  backend_resource_group_name: be_resource_group_name_foovalue_barvalue
+  backend_container_name: be_container_name_foovalue_{{ .Envs.BAR }}
+```