docs: Move developer docs from top-level to its own directory. (#1642)

docs/api-bucket-policy.md

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+## Access Policy
+
+This package implements parsing and validating bucket access policies based on Access Policy Language specification - http://docs.aws.amazon.com/AmazonS3/latest/dev/access-policy-language-overview.html
+
+### Supports following effects.
+
+    Allow
+    Deny
+
+### Supports following set of operations.
+
+    s3:GetObject
+    s3:ListBucket
+    s3:PutObject
+    s3:GetBucketLocation
+    s3:DeleteObject
+    s3:AbortMultipartUpload
+    s3:ListBucketMultipartUploads
+    s3:ListMultipartUploadParts
+
+### Supports following conditions.
+
+    StringEquals
+    StringNotEquals
+
+Supported applicable condition keys for each conditions.
+
+    s3:prefix
+    s3:max-keys
+
+### Nested policy support.
+
+Nested policies are not allowed.