README: linkify some paths

Change-Id: I77e31954f6d92f769934843bbba9f990f330f50a
Reviewed-on: https://go-review.googlesource.com/116675
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

README.md

@@ -19,14 +19,14 @@ BSD-style license found in the LICENSE file.
 Official binary distributions are available at https://golang.org/dl/.
 
 After downloading a binary release, visit https://golang.org/doc/install
-or load doc/install.html in your web browser for installation
+or load [doc/install.html](./doc/install.html) in your web browser for installation
 instructions.
 
 #### Install From Source
 
 If a binary distribution is not available for your combination of
 operating system and architecture, visit
-https://golang.org/doc/install/source or load doc/install-source.html
+https://golang.org/doc/install/source or load [doc/install-source.html](./doc/install-source.html)
 in your web browser for source installation instructions.
 
 ### Contributing