Fix filename endings in HACKING. Patch from "ckomlo", ticket #17515.

doc/HACKING/README.1st.md

@@ -5,20 +5,20 @@ In this directory
 This directory has helpful information about what you need to know to
 hack on Tor!
 
-First, read 'GettingStarted.txt' to learn how to get a start in Tor
+First, read 'GettingStarted.md' to learn how to get a start in Tor
 development.
 
 If you've decided to write a patch, 'CodingStandards.txt' will give
 you a bunch of information about how we structure our code.
 
-It's important to get code right!  Reading 'WritingTests.txt' will
+It's important to get code right!  Reading 'WritingTests.md' will
 tell you how to write and run tests in the Tor codebase.
 
 There are a bunch of other programs we use to help maintain and
-develop the codebase: 'HelpfulTools.txt' can tell you how to use them
+develop the codebase: 'HelpfulTools.md' can tell you how to use them
 with Tor.
 
-If it's your job to put out Tor releases, see 'ReleasingTor.txt' so
+If it's your job to put out Tor releases, see 'ReleasingTor.md' so
 that you don't miss any steps!
 
 
@@ -47,7 +47,7 @@ Happy hacking!
 
 XXXXX also describe
 
-doc/HACKING/WritingTests.txt
+doc/HACKING/WritingTests.md
 
 torguts.git