docs: Added docs.

docs/cli/envr_check.md

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+## envr check
+
+Check for missing binaries
+
+### Synopsis
+
+envr relies on external binaries for certain functionality.
+
+The check command reports on which binaries are available and which are not.
+
+```
+envr check [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for check
+```
+
+### SEE ALSO
+
+* [envr](envr.md)	 - Manage your .env files.
+