Rollup merge of rust-lang#64241 - adityaatluri:install-doc, r=Mark-Simulacrum

[doc] Added more prereqs and note about default directory

Author: Centril

README.md

@@ -32,6 +32,7 @@ or reading the [rustc guide][rustcguidebuild].
    * `cmake` 3.4.3 or later
    * `curl`
    * `git`
+   * `ssl` which comes in `libssl-dev` or `openssl-devel`
 
 2. Clone the [source] with `git`:
 
@@ -56,6 +57,8 @@ or reading the [rustc guide][rustcguidebuild].
     an installation (using `./x.py install`) that you set the `prefix` value
     in the `[install]` section to a directory that you have write permissions.
 
+    Create install directory if you are not installing in default directory
+
 4. Build and install:
 
     ```sh