Update Readme for MD5 -> SHA2 checksum change

README.md

@@ -148,8 +148,8 @@ $ cat fix1.patch fix2.patch | rbenv install --patch 1.9.3-p429
 
 ### Checksum verification
 
-If you have the `md5`, `openssl`, or `md5sum` tool installed, ruby-build will
-automatically verify the MD5 checksum of each downloaded package before
+If you have the `shasum`, `openssl`, or `sha256sum` tool installed, ruby-build will
+automatically verify the SHA2 checksum of each downloaded package before
 installing it.
 
 Checksums are optional and specified as anchors on the package URL in each
@@ -165,9 +165,9 @@ official URL specified in the definition file.
 You can point ruby-build to another mirror by specifying the
 `RUBY_BUILD_MIRROR_URL` environment variable--useful if you'd like to run your
 own local mirror, for example. Package mirror URLs are constructed by joining
-this variable with the MD5 checksum of the package file.
+this variable with the SHA2 checksum of the package file.
 
-If you don't have an MD5 program installed, ruby-build will skip the download
+If you don't have an SHA2 program installed, ruby-build will skip the download
 mirror and use official URLs instead. You can force ruby-build to bypass the
 mirror by setting the `RUBY_BUILD_SKIP_MIRROR` environment variable.