Update README

Author: pysysops

README.md

@@ -76,12 +76,43 @@ terraform-digitalocean-droplet:
 You can specify modules using Terraform's `source` specifications:
 https://www.terraform.io/docs/modules/sources.html
 
+### Versions
 The `version` can be a tag, a branch or a commit hash. By default, `xterrafile`
 will checkout the default branch of a module which is usually `master`.
 
-Modules will be downloaded to `./vendor/xterrafile/` by default.
+When using modules from a Terraform registry you can specify version ranges like:
 
-### Example Usage
+- `<1.0.0` Less than `1.0.0`
+- `<=1.0.0` Less than or equal to `1.0.0`
+- `>1.0.0` Greater than `1.0.0`
+- `>=1.0.0` Greater than or equal to `1.0.0`
+- `1.0.0`, `=1.0.0`, `==1.0.0` Equal to `1.0.0`
+- `!1.0.0`, `!=1.0.0` Not equal to `1.0.0`. Excludes version `1.0.0`.
+
+Note that spaces between the operator and the version will be gracefully tolerated.
+
+A `Range` can link multiple `Ranges` separated by space:
+
+Ranges can be linked by logical AND:
+
+  - `>1.0.0 <2.0.0` would match between both ranges, so `1.1.1` and `1.8.7` but not `1.0.0` or `2.0.0`
+  - `>1.0.0 <3.0.0 !2.0.3-beta.2` would match every version between `1.0.0` and `3.0.0` except `2.0.3-beta.2`
+
+Ranges can also be linked by logical OR:
+
+  - `<2.0.0 || >=3.0.0` would match `1.x.x` and `3.x.x` but not `2.x.x`
+
+AND has a higher precedence than OR. It's not possible to use brackets.
+
+Ranges can be combined by both AND and OR
+
+  - `>1.0.0 <2.0.0 || >3.0.0 !4.2.1` would match `1.2.3`, `1.9.9`, `3.1.1`, but not `4.2.1`, `2.1.1`
+
+
+### Module Download Location
+Modules will be downloaded to `./vendor/modules/` by default.
+
+## Example Usage
 Help:
 ```
 xterrafile help
@@ -96,7 +127,7 @@ Available Commands:
   version     Show version information for xterrafile
 
 Flags:
-  -d, --directory string   module directory (default "vendor/xterrafile")
+  -d, --directory string   module directory (default "vendor/modules")
   -f, --file string        config file (default "Terrafile")
   -h, --help               help for xterrafile