move stb_how.txt into docs directory

docs/git_how.txt

@@ -0,0 +1,11 @@
+git checkout working
+-- do work
+git add ...
+git commit
+
+to upload:
+
+git checkout master
+git pull ; usually unnecessary
+git merge working
+git push

tools/README.footer.md

@@ -40,7 +40,7 @@ Yes. I'll put a list here when people remind me what they are.
 
 #### Do you have any advice on how to create my own single-file library?
 
-Yes. https://github.com/nothings/stb/blob/master/stb_howto.txt
+Yes. https://github.com/nothings/stb/blob/master/docs/stb_howto.txt
 
 #### Why public domain?
 

mr.bat → tools/mr.bat

@@ -1,3 +1 @@
-cd tools
 debug\make_readme
-cd ..