Cleanup: add .txt extension to documentation

This will make it convenient as well as consistent as the other documentation that also uses the .txt extension.

diff --git a/build/documents/readme.bootstrap b/build/documents/readme.bootstrap.txt
similarity index 100%
rename from build/documents/readme.bootstrap
rename to build/documents/readme.bootstrap.txt

diff --git a/build/documents/readme.build b/build/documents/readme.build.txt
similarity index 100%
rename from build/documents/readme.build
rename to build/documents/readme.build.txt

diff --git a/build/documents/readme b/build/documents/readme.txt
similarity index 100%
rename from build/documents/readme
rename to build/documents/readme.txt