Merge pull request #77 from zchee/readme_file-link

README: add file link and fix profile.proto file path

README.md

 profile.proto is a protocol buffer that describes a set of callstacks
 and symbolization information. A common usage is to represent a set of
 sampled callstacks from statistical profiling. The format is
-described on the src/proto/profile.proto file. For details on protocol
+described on the [proto/profile.proto](./proto/profile.proto) file. For details on protocol
 buffers, see https://developers.google.com/protocol-buffers
 
 Profiles can be read from a local file, or over http. Multiple
 
 ## Further documentation
 
-See doc/pprof.md for more detailed end-user documentation.
+See [doc/pprof.md](doc/pprof.md) for more detailed end-user documentation.
 
-See doc/developer/pprof.dev.md for developer documentation.
+See [doc/developer/pprof.dev.md](doc/developer/pprof.dev.md) for developer documentation.
 
-See doc/developer/profile.proto.md for a description of the profile.proto format.
+See [doc/developer/profile.proto.md](doc/developer/profile.proto.md) for a description of the profile.proto format.