Add more info about .pioenvs directory

docs/projectconf.rst

@@ -97,7 +97,11 @@ This option can be overridden by global environment variable
 ``envs_dir``
 ^^^^^^^^^^^^
 
-This is a cache directory. *PlatformIO Build System* uses this folder for project
+.. warning::
+    **PLEASE DO NOT EDIT FILES IN THIS FOLDER**. PlatformIO will overwrite
+    your changes on the next build. **THIS IS A CACHE DIRECTORY**.
+
+*PlatformIO Build System* uses this folder for project
 environments to store compiled object files, static libraries, firmwares and
 other cached information. It allows PlatformIO to build source code extremely
 fast!

platformio/util.py

@@ -81,6 +81,7 @@ class cd(object):
 
 
 class memoized(object):
+
     '''
     Decorator. Caches a function's return value each time it is called.
     If called later with the same arguments, the cached value is returned
@@ -212,10 +213,18 @@ def get_projectlib_dir():
 
 
 def get_pioenvs_dir():
-    return _get_projconf_option_dir(
+    path = _get_projconf_option_dir(
         "envs_dir",
         join(get_project_dir(), ".pioenvs")
     )
+    dontmod_path = join(path, "do-not-modify-files-here.url")
+    if not isfile(dontmod_path):
+        with open(dontmod_path, "w") as fp:
+            fp.write("""
+[InternetShortcut]
+URL=http://docs.platformio.org/en/latest/projectconf.html#envs-dir
+""")
+    return path
 
 
 def get_projectdata_dir():